<div id="readability-page-1">You might not realize it, but Thursday, Aug. 14, was arguably the most important day of the quarter for investors. Although earnings season can clue investors in to the health of Wall Street's leading businesses, Form 13Fs are equally valuable. A 13F is a required filing due no later than 45 calendar days following the end of a quarter (Aug. 14) for institutional investors with at least $100 million in assets under management. It provides investors with a concise snapshot of which stocks Wall Street's smartest and most successful money managers bought and sold in the latest quarter (in this instance, the quarter that ended in June). While Wall Street is packed with brand-name asset managers, none garners the attention of investors quite like Berkshire Hathaway (BRK.A -0.26%) (BRK.B -0.35%) CEO Warren Buffett. Mirroring the aptly dubbed Oracle of Omaha's buying and selling activity has been a time-tested strategy for wealth creation. <div><img src="https://imageproxy.pbkrs.com/https://g.foolcdn.com/image//query-b3A9cmVzaXplJnVybD1odHRwczovL2cuZm9vbGNkbi5jb20vZWRpdG9yaWFsL2ltYWdlcy84Mjk1MjMvYnVmZmV0dDgtdG1mLmpwZyZ3PTcwMA?x-oss-process=image/auto-orient,1/interlace,1/resize,w_1440,h_1440/quality,q_95/format,jpg" width="700" height="466" original-src="https://imageproxy.pbkrs.com/https://g.foolcdn.com/image//query-b3A9cmVzaXplJnVybD1odHRwczovL2cuZm9vbGNkbi5jb20vZWRpdG9yaWFsL2ltYWdlcy84Mjk1MjMvYnVmZmV0dDgtdG1mLmpwZyZ3PTcwMA"/>Berkshire Hathaway CEO Warren Buffett. Image source: The Motley Fool. </div> Although every facet of Buffett's trading activity tends to be put under the microscope by professional and everyday investors, this quarterly 13F was unique. It revealed the mystery stock that Berkshire's billionaire boss had been buying under the guise of secrecy -- and it's not what anyone on Wall Street had been expecting. <h2>Warren Buffett buys big under the &#34;confidential treatment&#34; tag</h2> On rare occasions, regulators grant prominent money managers the ability to build up a stake in one or more securities under the &#34;confidential treatment&#34; tag. Confidential securities don't need to be reported on a 13F, and there's a purpose behind this. When a high-profile asset manager like Warren Buffett opens a position in a stock, it's not uncommon for its share price to surge as investors pile in. Being granted confidential treatment allows these prominent investors (like Buffett) to build up a position at a more attractive cost basis. The Oracle of Omaha has been granted the confidential treatment tag on a couple of occasions over the last decade: <ul> <li>In 2015, Berkshire was afforded this exclusion to build up a stake in energy giant Phillips 66.</li> <li>During the early stages of the COVID-19 pandemic, the confidential treatment tag was used by Buffett and his team to create multibillion-dollar positions in Verizon Communications and Chevron, the latter of which remains a core holding.</li> <li>In the second half of 2023 and during the first quarter of 2024, this special tag was used to pile into property &amp; casualty insurer Chubb.</li> <li>It was also granted during the first half of 2025 for a mystery stock that's now been revealed following Berkshire's second-quarter 13F filing.</li> </ul> Though there are thousands of publicly traded companies, Berkshire Hathaway's quarterly operating results have offered big clues as to where this mystery stock resides. Every quarter, Buffett's company provides a cost basis breakdown of the securities held in its investment portfolio from three categories: <ul> <li>Banks, insurance and finance</li> <li>Consumer products</li> <li>Commercial, industrial and other</li> </ul> The cost bases for the first two segments have been declining, while commercial, industrial and other has been climbing. In other words, it was pretty clear that Buffett's mystery buy wasn't a financial stock or consumer staples company. We also know that, historically speaking, Berkshire's billionaire chief tends to avoid the tech and healthcare sectors. Buffett isn't the most tech-savvy, and keeping up on clinical-trial data isn't high on his list of things to do. All signs were pointing to an industrial stock being his big purchase during the first half of 2025...but Wall Street was way off! <div><img src="https://imageproxy.pbkrs.com/https://g.foolcdn.com/image//query-b3A9cmVzaXplJnVybD1odHRwczovL2cuZm9vbGNkbi5jb20vZWRpdG9yaWFsL2ltYWdlcy84Mjk1MjMvc3RvY2stbWFya2V0LWNhbmRsZXN0aWNrLWNoYXJ0LWNpcmNsZS1idXktZGlwLWludmVzdC1nZXR0eS5qcGcmdz03MDA?x-oss-process=image/auto-orient,1/interlace,1/resize,w_1440,h_1440/quality,q_95/format,jpg" width="700" height="486" original-src="https://imageproxy.pbkrs.com/https://g.foolcdn.com/image//query-b3A9cmVzaXplJnVybD1odHRwczovL2cuZm9vbGNkbi5jb20vZWRpdG9yaWFsL2ltYWdlcy84Mjk1MjMvc3RvY2stbWFya2V0LWNhbmRsZXN0aWNrLWNoYXJ0LWNpcmNsZS1idXktZGlwLWludmVzdC1nZXR0eS5qcGcmdz03MDA"/>Image source: Getty Images. </div> <h2>The Oracle of Omaha lifts the curtain on his mystery stock</h2> Even though Berkshire's boss has been a persistent net seller of stocks for 11 consecutive quarters, to the tune of $177.4 billion, he did quite a bit of buying during the quarter ending in June. Berkshire's 13F shows six new positions were opened, with a half-dozen existing holdings being added to. However, the one new purchase that's bound to raise eyebrows and drop jaws is the reveal of the mystery stock: UnitedHealth Group (UNH 12.04%). Buffett oversaw the purchase of 5,039,564 shares of UnitedHealth, worth close to $1.6 billion at the end of June. There's not a sector Warren Buffett understands better or feels more comfortable investing in than financials. While banks and credit service providers might be the first that come to mind when hearing the word &#34;financials,&#34; it also includes an assortment of insurance companies, such as Chubb. A substantial portion of UnitedHealth's business involves health insurance. Though UnitedHealth isn't protecting property, the premise of insurance companies is similar across the board. They collect premiums and outlay capital to cover member claims, with the expectation being that premiums collected will far outweigh the amount spent on its members. Typically, insurers possess exceptional pricing power. Since catastrophes and higher claim periods are inevitable, insurers are often justified in raising their premiums on a near-annual basis. When insurers aren't outlaying their collected premium to cover claims, it's invested in safe, interest-bearing assets to bolster profits. But UnitedHealth Group is more than just a leading health insurance company. Its healthcare services segment, Optum, has been a faster-growing and higher-margin counterpart for quite some time. Optum provides healthcare companies with everything from care delivery and pharmacy care services to the software they use. Warren Buffett's modus operandi is to wait for price dislocations to present themselves. In a historically pricey stock market, locating value has been virtually impossible. Yet, with UnitedHealth Group stock losing more than half its value since the midpoint of April, a rare price dislocation has emerged. However, UnitedHealth stock didn't dive 54% on a whim. Its poor performance is a reflection of higher costs associated with the company's Medicare Advantage segment (also known as Medicare Part C). Whereas many of UnitedHealth's peers anticipated higher expenses in this private Medicare segment in 2025, UnitedHealth did not and is now paying the price. The good news is that UnitedHealth Group has endured challenging environments like this before, and they rarely last for any significant length of time. The company should be able to address premiums for the upcoming year and work to lower its costs. UnitedHealth Group's forward price-to-earnings (P/E) ratio of 14 (prior to Buffett revealing it as his mystery stock) represents a 26% discount to its average forward-year earnings multiple over the trailing-five-year period. It's also a company with a robust capital-return program, highlighted by a growing dividend and hearty share buybacks. Though a healthcare stock is probably the last thing Wall Street was expecting, UnitedHealth Group checks all the right boxes for the value-oriented Oracle of Omaha. </div>

Warren Buffett's recent 13F filing revealed a surprising new investment: UnitedHealth Group, with Berkshire Hathaway purchasing over 5 million shares valued at approximately $1.6 billion. This marks a significant shift as Buffett has been a net seller of stocks for 11 quarters. The filing also highlighted the use of 'confidential treatment' for certain stock purchases, allowing Buffett to build positions without immediate market impact. The revelation of UnitedHealth as the mystery stock contrasts with expectations of an industrial focus, showcasing Buffett's strategic investment in the healthcare sector.

Warren Buffett's Mystery Stock Is Revealed, and It Comes as a Big Surprise to Wall Street