Bill Ackman's Pershing Square Capital Management gained 7.9 percent net of fees for the month of October in its largest fund, dramatically reversing the year-to-date returns of the outspoken activist investor.
Pershing Square International is now up 8.1 percent for the year, according to an investor performance update obtained by CNBC.com. The same fund had been up just 0.2 percent through September.
(Read more: Pershing Square to sell 6M Canadian Pacific shares)
Pershing Square now manages $11.44 billion as of November 1 versus $10.77 billion a month before.
The letter also noted a second undisclosed short position. Ackman is already engaged in a highly publicized battle with nutritional supplement company Herbalife, which he believes is a pyramid scheme and announced a $1 billion short position against the stock in December. Shares have risen all year, costing the fund hundreds of millions of dollars in paper losses.
It wasn't immediately clear what drove the large returns or what the short position was. Ackman declined to comment.
—By CNBC's Lawrence Delevingne and Maneet Ahuja. Follow them on Twitter: @ldelevingne and @wallstmaneet