What: Shares of Medivation (NASDAQ:MDVN), biopharmaceutical company primarily focused on treating cancer, were down more than 10% today on heavy volume after the company reported its third quarter results.

So what: Revenue during the quarter came in at $260.7 million, which was up 30% over the year ago period and was much higher than Wall Street's forecast of only $229.1 million. Non-GAAP net income of $58.4 million, or $0.35 per share, also managed to come in well ahead of the $0.33 per share that analysts were expecting.

Sales of Xtandi, which treats metastatic castration-resistant prostate cancer, were strong in the U.S. and abroad. Medivation's partner Astellas Pharma reported huge growth in sales of Xtandi -- 73% and 71% growth, respectively, over the year-ago period. 

As a reminder, Medivation generates revenue from its partnership related to sales of Xtandi in three different ways: U.S. collaboration revenue, ex-U.S. collaboration revenue, and upfront and milestone payments.

Here's how those results shook out:

  Q3 2015 Q3 2014 Change %
U.S. Collaboration: $156.5 million  $90.7 million 73%
Ex-U.S. Collaboration $33.6 million $15.5 million 116%
Milestone Payments $70.6 million  $94.2 million -25%

As you can see, collaboration revenue from sales of Xtandi was up strong across the board, though milestone payments, which are lumpy by nature, were down year over year.

For the full year, the company reiterated its financial guidance, which breaks down like this:

Now what: I must admit that I'm left scratching my head as to why shares were down today. Perhaps Mr. Market was expecting the company to raise its guidance like they did last quarter, and the fact they only reiterated it gave investors pause. Sometimes you just never know, and either way shares have now declined nearly 20% year to date. 

In the press release, CEO David Hung stated:

"During the third quarter Medivation made significant progress led by continued demand growth for XTANDI for the treatment of metastatic castration-resistant prostate cancer. In October, we successfully completed the acquisition for the late stage asset talazoparib, a highly potent, orally available poly ADP ribose polymerase (PARP) inhibitor. With the acquisition of talazoparib, we now have further diversified our late-stage pipeline beyond XTANDI and pidilizumab to complement our early development programs, providing significant opportunities for growth."

Medivation continues to perform well, so today's sell-off on good new certainly looks like a buying opportunity to me.

Brian Feroldi has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.