Activist investor William Ackman's Pershing Square Holdings portfolio tumbled 18.6 percent so far this year, according to an investor with knowledge of the numbers.
The loss comes after Pershing Square ended 2015 with its largest ever annual loss.
Ackman said in a recent letter that many big investors who had followed him into stocks were now selling out as the markets tumble. He did not disclose details.
The fund ended 2015 with a 20.5 percent loss after posting a 40 percent gain in the previous year, which cemented Ackman's tenure as one of the hedge fund industry's biggest stars.
