One day you're hot. The next day you're not.

Such is the life of a momentum stock, which begins to explain why ImClone Systems ( IMCL) shares were trading above $80 on Tuesday but plummeted 18.9% Wednesday to close at $65.44, despite what appeared to be strong second-quarter results.

Blame the dreaded "whisper" number (whatever that is) for ImClone's downfall. Going into Wednesday morning, investors wanted to see ImClone report second-quarter Erbitux sales of $80 million-plus. Printed estimates of the cancer drug's sales from sell-side analysts were far lower, but the only number that was going to matter was the unprinted -- and much higher -- Erbitux forecast.

ImClone reported actual second-quarter Erbitux sales of $71.4 million. It was a miss, a disappointment, a letdown for the fast-money crowd that had bid the stock up in anticipation of better news. The typical sell-on-the-news reaction that might have taken ImClone down a couple of dollars quickly snowballed into a massive selloff.

ImClone closed Wednesday down $15.29 from Tuesday's close of $80.73. More than 23 million shares traded hands, seven times the average volume.

Looking at it another way, an Erbitux "miss" of less than $10 million was responsible for the shedding of more than $1 billion in market value (even though Erbitux sales were actually quite good and second-quarter EPS of 29 cents beat consensus estimates by 4 cents).

"ImClone is a sentiment-driven stock in a sentiment-driven market, and today, you saw sentiment turn on a dime," says one biotech money manager with no position in the stock.

ImClone then proceeded to take the entire biotech sector down with it. The Amex Biotech Index shed 6% to 461, a closing level not visited since last December. The Biotech HOLDRs Trust ( BBH) fell 3%.

Other things bothered investors about ImClone today. On Tuesday night, the company disclosed that it would seek a second approval for Erbitux to treat head and neck cancer, but that the filing with the Food and Drug Administration wouldn't be ready until the second quarter of 2005. Most analysts and investors were counting on the filing to be completed in the fourth quarter of this year.

ImClone has never provided timeline guidance for an Erbitux head and neck cancer filing, and said Wednesday that data from two completed and positive clinical trials was still being analyzed and reviewed, which explained why the filing would be pushed into next year.

On its conference call Wednesday, ImClone executives did not provide forward sales guidance for Erbitux. This is not an uncommon practice in biotech: Genentech ( DNA), for instance, has never provided top-line guidance. But on a day when ImClone stock was already weak, guidance, particularly strong guidance, might have been viewed as some sort of safety net. No such support materialized, however.

Let's face it: ImClone is a stock that has soared eight times in two years. It's always been driven by momentum and emotion. Valuation? That was something for other stocks to worry about, not ImClone -- at least until there was no other reason left to own the stock.

Friedman Billings Ramsey biotech analyst Jim Reddoch downgraded ImClone Wednesday to market perform from outperform, dropping his price target to $75 from $93. The title of his note is "Erbitux sales not enough, catalyst moved out."

That's all you really need to know. ImClone had a nice run while it lasted. No doubt it will run again.
Adam Feuerstein writes regularly for RealMoney.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. He invites you to send your feedback to adam.feuerstein@thestreet.com.