By Steven Russolillo, and David Benoit 

Five hours and a dozen presentations into the annual Sohn Investment Conference, a speaker finally drew a big market reaction, as hedge-fund manager David Einhorn unveiled a bet against athenahealth Inc.

Shares of the electronic health-care-records provider plummeted after Mr. Einhorn, founder of Greenlight Capital Inc., said the stock may fall 80%.

One of the few hedge-fund managers to relish talking about his shorts, Mr. Einhorn said athena and other technology stocks were indicative of a bubble. Shares in the company, which wasn't available for comment, quickly dropped 10% to about $113 in after-hours trading.

"The thing about bubble stocks is the best reason to own them is because they are going up," Mr. Einhorn said of the company and other tech stocks that have risen in recent months. "When they stop going up, these stocks become falling knives."

Mr. Einhorn had the biggest impact during a day of recommendations from high-profile hedge-fund managers on topics from the U.S. housing market to China's currency to the Brazilian presidential race.

Paul Tudor Jones, a 34-year hedge-fund veteran who counts as part of the old guard in the industry, took a broader view, calling the trading environment "as difficult as I've ever seen it in my career."

Mr. Jones's main fund is down about 4% this year, according to investor documents. The billionaire said he has been particularly perplexed by a lack of interest-rate movement that has hurt his ability to make relative- value investments, bets on assets appreciating while wagering on others depreciating. So-called macro funds generally make investments based on global financial trends rather than individual stocks. "What we desperately need is a macro doctor to proscribe central bank Viagra because otherwise it is going to continue to be somewhat dull," he said at the annual gathering of hedge-fund managers.

Macro funds have lost money three years running, says research firm HFR Inc. Emerging- market hedge funds dropped about 1% in the first quarter.

One manager whose fund has been hurt lately, Michael Novogratz, who co-manages a macro fund for Fortress Investment Group LLC, said he sees a bottom nearing in Brazil, especially if Brazil President Dilma Rousseff loses a re-election bid this fall. "It's so bad, it's good," he said.

A former Julian Robertson protégé, Chris Shumway, who closed his own eponymous shop three years ago, encouraged investors to short China's offshore currency.

"Our view is they have limited options within China to deal with their slowing growth," Mr. Shumway said.

Bill Ackman, the founder of Pershing Square Capital Management LP, went to new lengths to attack the U.S. Senate plan to wind down and overhaul the government-run mortgage-finance companies Fannie Mae and Freddie Mac.

In his first extended public remarks since accumulating nearly 10% of the firms' common shares starting in November, Mr. Ackman said Fannie and Freddie were essential to preserving the housing market. He expressed doubt that lawmakers could raise the $500 billion needed to capitalize the firms in any new iteration.

"We don't think there's an investor in the world of any consequence that will invest in the new version of Fannie and Freddie," he said.

He wasn't the only investor with a provocative view on housing. Bond-fund manager Jeffrey Gundlach of DoubleLine Capital LP pointed to a generational shift away from home buying and toward renting, and recommended shorting, or betting against, an exchange-traded fund that tracks the prospects of major home builders.

"Single-family housing is overbelieved and overrated," he said.

Despite the bearish picks, the mood was often light inside the packed Avery Fisher Hall, normally home to the New York Philharmonic. Mr. Jones said he slips into "daydreaming about winning 'Dancing with the Stars' on some days in the office."

James Grant, editor of Grant's Interest Rate Observer, brought a bit of topical humor to his presentation, boosting Russian state-controlled natural-gas giant OAO Gazprom. He said the stock had been so beaten down it was "Donald Sterling with a London ticker," a reference to the Los Angeles Clippers owner who was banned from the National Basketball Association last week after recordings of his racist comments went public.

Some investors went for dry jokes. "There's an alarming outbreak of old people in the United States," observed Larry Robbins of Glenview Capital Management in his pitch for Medicare-focused insurer Humana Inc.

Although there were many billions of dollars of net worth represented on stage, profitability of the managers' picks was far from certain. In a bulletin to clients, Westport, Conn., research firm Birinyi Associates Inc. warned that the investment ideas presented at last year's conference underperformed the Standard & Poor's 500 by 19.1 percentage points.

An investor who wagered an equal amount of money on all of last year's Sohn picks would have been down 3.8% by now, compared to a 15.2% for the S&P, Birinyi said.

Proceeds from the Sohn Conference support research programs for the treatment and cure of pediatric cancer. Organizers said they have raised more than $50 million for the cause since 1995.

Rob Copeland and Sarah Krouse contributed to this article.

Write to Rob Copeland at rob.copeland@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires

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