Biogen Idec (NASDAQ:BIIB) reported nice solid growth on the bottom line yesterday -- earnings per share were up 54% year over year -- but it’s the top line that investors should be worried about.

The impressive earnings growth shouldn't be discarded. After all, earnings and cash flow are what investors actually "get" out of owning shares of the company. Larger companies like Johnson & Johnson (NYSE:JNJ) and Procter & Gamble (NYSE:PG) have rewarded investors by growing the bottom line faster than the top. But, for growth companies like Biogen, increases in revenue paint a picture of what the company may look like in the future.

And it isn't a rosy picture right now. It's more along the lines of a piece of abstract art that's difficult to interpret. In the first quarter, revenue was up just 10%, compared to 29% revenue growth between 2007 and 2008.

Revenue from sales of multiple sclerosis drug Tysabri, which Biogen sells with Elan (NYSE:ELN), was up 44%, but the patients coming onto the drug in the first quarter slowed from the fourth quarter of last year. In order to convert large numbers of patients from other medications -- like Teva Pharmaceutical's (NASDAQ:TEVA) Copaxone, or Rebif from EMD Serono and Pfizer (NYSE:PFE) -- Biogen and Elan need to convince patients that the superiority of Tysabri is worth the risk of getting a potentially deadly brain infection called progressive multifocal leukoencephalopathy (PML). That's undoubtedly no easy task, but they're giving it their best shot, although it's probably too early to know if the benefits of Tysabri's makeover are having an effect.

Sales of Biogen's other two drugs also weren't all that impressive. Sales of its other multiple sclerosis drug, Avonex, were up just 3.6%, and cancer treatment Rituxan, which it sells with Genentech, managed to grow just 6%. In a normal economy, that would be downright pathetic, but other mid-sized drug companies like Celgene (NASDAQ:CELG) have shown that health care is still susceptible to a weak economy. So Biogen's lackluster growth is understandable, even if investors aren't happy about it.

The problem with the lack of growth is that it introduces an air of caution. Until Biogen shows that it can get back to hyperactive revenue growth, investors are likely to assign a lower multiple to the stock.

Biogen is an active Stock Advisor pick. Click here to start a 30-day risk-free trial of the newsletter and see all the Gardner brothers' current recommendations.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Elan is a Rule Breakers recommendation. Pfizer is an Inside Value pick. Johnson & Johnson and Procter & Gamble are Income Investor picks. The Fool owns shares of Procter & Gamble and has a disclosure policy.