Longleaf Partners Fund, managed by Southeastern Asset Management, has released its investor letter for the fourth quarter of 2025. The fund returned 3.35% during the quarter. This compares to a 2.66% return for the S&P 500 and a 3.81% return for the Russell 1000 Value Index.
The firm described 2025 as a challenging year without standout performers. About 5% of its portfolio gained 20% or more, while 35% of the S&P 500 achieved that level. The overall market dynamic was cited for the fund's relative underperformance.
The letter reiterated a focus on actions to strengthen portfolio outcomes rather than chasing winners at the wrong time. It stated that building a portfolio of real companies during a period of excessive speculation will benefit all markets.
In the letter, the fund highlighted its holding in Regeneron Pharmaceuticals, Inc. (NASDAQ:REGN). The biotechnology company is headquartered in Tarrytown, New York. On January 16, 2026, its stock closed at $733.04 per share. Its one-month return was -5.94%, while it gained 7.55% over the last 52 weeks. The company has a market capitalization of $77.693 billion.
Regarding Regeneron, the fund stated, "Regeneron Pharmaceuticals, Inc. (NASDAQ:REGN) – Healthcare company Regeneron was a detractor in the first half of the year but ended up as a contributor for the quarter and full year. It did not initially feel great, but our phased buy-in has worked well so far. We continue to like the company’s strong position in allergic diseases (DUPIXENT) and oncology (LIBTAYO and others). We believe the market focuses too much on EYLEA, a retinal disease medication which is less than 20% of the company's value. The company has a net cash balance sheet and great owner-partners. Unlike most others in its industry, it has sworn off large M&A and was a material share repurchaser in 2025. We still are not paying much (or anything depending on how you do the math) for the company’s sizeable pipeline value. After some initial pipeline disappointment in the first part of the year, there were more positives as the year went on. The P/V remains low, and we are excited for Regeneron to be a meaningful position entering the year."