(FORTUNE Magazine) – What you don't know can hurt you. Especially when you used to know it. What we're talking about here is disclosure, or rather the lack of it. In many instances, companies suddenly stop disclosing vital bits of data--right before business takes a dive. Consider Hewlett-Packard, a company with a history of giving out reams of information. Fred Hickey, editor of the High-Tech Strategist newsletter, particularly liked the way HP disclosed orders by category during its conference calls. Enter CEO Carly Fiorina, vowing to be even more open. Shortly after her arrival in July 1999 she held her first conference call, and what was missing? You guessed it: orders by category. "That," Hickey says, "should've been a tip-off to investors right there." Within months, HP's stock had sunk from $44 to $30. A string of earnings disappointments--not to mention its unpopular plan to buy Compaq--have since pushed the stock to around $17.

So the trick, it would seem, is to find companies that have gone mum. (Because, like Mum said, if you don't have anything good to say, don't say anything at all.) I sent out an all-points bulletin to the best hedge fund managers, analysts, and short-sellers in my Palm Pilot. The result: a surprising list of the suddenly silent.

Start with motorcycle maker Harley-Davidson. Until early this year, the Motorcycle Industry Council published monthly sales data, broken down by brand. In March, several news stories used these numbers to point out that Harley's sales were slowing. Soon afterward, the trade group switched to quarterly reporting instead. Harley doesn't deny encouraging this change. "We don't think monthly data is very meaningful," says spokesman Pat Davidson. Maybe not, but Banc of America Securities analyst Gary Cooper confesses to missing what he thought was "a great data point."

Monthly sales data is increasingly falling out of favor with retailers as well. Following the lead of Circuit City and Best Buy, Radio Shack plans to switch from monthly to quarterly sales data at the end of the year. A Radio Shack spokeswoman concedes the company hopes this change will help make its stock less volatile. (It didn't do much for Circuit City: The stock trades for $16, down from $65 in April 2000.)

Then there's software maker SmartForce, which once proudly announced its rising backlog in both its 10-Q and its press releases. The disclosure stopped, without explanation, last March. SmartForce officials couldn't be reached for an explanation; to their credit, the company has yet to report bad news. If it does, you can bet investors will wonder whether a declining backlog would have been a leading indicator.

Finally, I came across a company that readily fessed up to shutting up--but blamed its customers for its doing so. Until this year's second quarter, optical-components maker Finisar ran down its top ten customers in its quarterly conference call. That information is a gold mine to investors keeping tabs on the fortunes of Finisar--and its customers. "Whenever there was a change, our customers got barraged with phone calls," says Finisar CFO Steve Workman. Finisar stopped giving out this information as of its last conference call. Says Workman: "At the end of the day, it is better to keep customers and make them happy." Even at the expense of investors.

Herb Greenberg is a senior columnist for TheStreet.com. Questions? Comments? Contact him by e-mail at herb@thestreet.com.

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