Bill Ackman is having the worst year in his fund's history

Hedge fund billionaire Bill Ackman, founder of $12 billion Pershing Square Capital, is having the worst year in his fund's history.

Pershing Square Holdings, his fund's publicly traded vehicle, has fallen 26.4% through March 15, according to a performance update.

The first quarter isn't even over.

Last year, Pershing Square suffered its "worst year" in its history, losing 20.5%.

His returns so far this year have been mostly dragged down by Valeant Pharmaceuticals' massive decline. Ackman is the largest hedge fund shareholder with more than 30.71 million shares, or a 9% stake.

Ackman lost more than $1 billion (on paper) on his position on Tuesday alone. Since disclosing a position in the first quarter of 2015, his fund's losses to date on his Valeant investment are estimated at north of $2.5 billion.

Valeant got slaughtered on Tuesday after the company reported fourth-quarter earnings results that fell below analysts expectations and cut its revenue forecast for the year. The stock collapsed 51% on the news.

The lowered revenue guidance adds to a tough run for Valeant, which has been sliding since late 2015 because of scrutiny in Washington, D.C., over drug-price increases and accusations from a short seller. Just last month, the company confirmed that it was part of several ongoing investigations.

Ackman, a well-known activist investor, makes large, concentrated bets in a handful of companies. This year, the majority of his long positions have collapsed, while his only short bet, Herbalife, has made gains.

Ackman also announced Wednesday that he had sold a big portion of his Mondelez stake.

Here's a rundown of how his stock holdings have performed this year/ so far in March:

• Valeant (VRX): -68% YTD/ -49% MTD
• Platform (PAH): -38% YTD/ +12% MTD
• Fannie Mae (FNMA): -7.3% YTD/ +10.22% MTD
• Freddie Mac (FMCC): -9.3% YTD/ +8.21% MTD
• Canadian Pacific (CP): -0.48% YTD/ +4.79% MTD
• Howard Hughes (HHC): -13.64% YTD/ +5.27% MTD
• Restaurant Brands International (QSR): -3.44% YTD/ +5.29% MTD
• Air Products & Chemicals (APD): -0.4% YTD/ +1.17% MTD
• Zoetis (ZTS): -15% YTD/ -0.8% MTD
• Mondelez (MDLZ): -7.14% YTD/ +2.74% MTD

Meanwhile, Herbalife, a multilevel marketer that sells weightloss shakes, has climbed more than 7% this year. Herbalife has gained more than 5.3% this month.

Ackman has been betting against the company since 2012 on the belief that it's operating a "pyramid scheme" that preys upon poor people. Herbalife has denied his allegations.