This quarter, most Biogen Idec (NASDAQ:BIIB) investors had their eyes on the sales and prescription numbers for the return of multiple sclerosis (MS) drug Tysabri, now back on the market in partnership with Elan (NYSE:ELN). Yet the results of Biogen's other MS drug, Avonex, posted the most surprising results.

Avonex sales climbed an unexpected 19% year over year for the quarter, easily its highest rate of growth in the past year.

Quarter

Avonex sales*

Y-O-Y growth

Q3 2006

$445

19%

Q2 2006

$429

12%

Q1 2006

$393

5%

Q4 2005

$413

12%
*In millions.

These soaring Avonex sales, plus a solid quarter of 12% sales growth for the cancer drug Rituxan, helped overall revenues increase 18% to $703 million for the quarter. Rituxan was just approved to treat an additional two new indications in September.

Earnings, excluding amortization charges, were up 70% to $207 million, or $0.60 a share. Adjusted operating margins were an impressive 37%, scaling up significantly from 16% in last year's third quarter.

This was Biogen's largest year-over-year percentage gain in revenues since the fourth quarter of 2004. Investors' surprise at the good results was evident in the rise in Biogen shares yesterday of more than 8%. The fairly modest relaunch of Tysabri certainly wasn't the key catalyst for investors' renewed enthusiasm.

Even with guidance for 2006 non-GAAP earnings per share raised to more than $2.20, Biogen shares are trading at a reasonable 24 times earnings, considering the level of scale and sales growth it has exhibited this year. Even if Avonex sales can't continue to increase at this pace, the ramping up of Rituxan and even Tysabri revenues should provide years of growth for Biogen.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy .