For drugmakers like Exelixis (Nasdaq: EXEL) that specialize in developing compounds to fight solid tumors, there is no bigger show for their top clinical trial data than the American Society of Clinical Oncology (ASCO) meeting in late May or early June.

Exelixis will present analyses of study data at ASCO for all of its lead cancer-fighting compounds. Phase 1 data for two of Exelixis' more exciting compounds, XL184 and XL765, will be included, as well as more phase 2 data for XL647 and XL880, which were developed in-house -- GlaxoSmithKline (NYSE: GSK) took over development of XL880 as a partner in December.

What makes ASCO so important is that many doctors and key opinion leaders in oncology use the event to help form their opinions on these anti-cancer drugs. Drugmakers like ImClone Systems (Nasdaq: IMCL) and Onyx Pharmaceuticals (Nasdaq: ONXX) often save their best and most exciting clinical trial data for ASCO. This is why frustrated investors often only get to hear "study failed" or "study succeeded" when a drugmaker releases results of a clinical study before a medical conference.

As one biotech executive said, "oncology is a data-driven market," so a drug's success or failure can often depend on the data that a company (and its competitors) showcase at ASCO and the other major conferences. ASCO is not set to begin until May 30 this year, but all investors will have an idea about the quality of the clinical trial data that will be released there.

ASCO will publicly post all of the abstracts for its meeting on its website at 9 p.m. Eastern time on May 15. Expect to see some extreme volatility in Exelixis' and all other ASCO-related drugmakers' stocks as investors digest the data.

Exelixis is recommended in our market-beating Rule Breakers newsletter, and The Motley Fool owns shares of the company. GlaxoSmithKline is an Income Investor pick.

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