The shift of fortunes shows how much tumbling markets, sparked by sharp declines in Chinese stock prices, have
hurt the world's most prominent investors.
"At the date of this report, the year to date investment performance has been erased, and the Company is at a loss position for the year," Ackman's Pershing Square Capital Management said in its interim financial statement released to investors on Wednesday afternoon.
Only three weeks ago the New York-based manager had told clients, including state pension funds in Massachusetts and New Jersey, that the fund was up 10.1 percent for the year through the end of July.
Ackman is one of the first major money managers to tell his investors how the market turmoil has hit his firm, but he did not say how much money the fund has lost this year.
He and other hedge fund managers are expected to release monthly performance numbers next week.
Despite the sharp market moves, Pershing Square, which holds only a small number of investments at a time, has "made no meaningful recent changes to (our) current portfolio holdings," Ackman said in the financial statement.
Lower commodity prices or economic weakness in China are unlikely to have a "material impact on the intrinsic value of the portfolio," Ackman added.
Instead, he expects demand for cookies, industrial gases, specialty pharmaceutical products, and animal health products made by companies in his portfolio such as Mondelez International Inc (>> Mondelez International Inc), Air Products and Chemicals Inc (>> Air Products & Chemicals, Inc.), Valeant Pharmaceuticals International Inc (>> Valeant Pharmaceuticals Intl Inc) and Zoetis Inc (>> Zoetis Inc) to remain "robust over the long-term."
In the first six months of the year, Allergan, Valeant, Nomad Foods and Mondelez were the fund's biggest winners. Meanwhile Herbalife Ltd (>> Herbalife Ltd.), which he has made a $1 billion short bet against, cost the fund 3.7 percent in returns.
Due to recent strong returns, the fund's size had swelled to roughly $20 billion, but that is set to shrink in light of the latest drop. Ackman did not put a number on that decline.
Last year Pershing Square gained 40.4 percent in value while the Standard & Poor's 500 index rose 13.7 percent.
(Reporting by Svea Herbst-Bayliss; Editing by Bill Rigby)
By Svea Herbst-Bayliss