Genentech said Tuesday that a new clinical trial had shown that its best-selling drug Avastin was effective in treating breast cancer, a finding that could increase the chances that the product will be approved for that use.

The company said in an announcement after the close of trading that the trial showed that Avastin lengthened the time before the cancer worsened. Genentech’s shares rose 2.4 percent, to $71.60, in after-hours trading.

The Food and Drug Administration is scheduled to decide by Feb. 23 whether to approve Avastin for use in treating breast cancer. The decision is being closely watched as a barometer of the agency’s standards toward approving new cancer drugs or expanding uses of current medications. It is possible the F.D.A. will delay the decision past February so it can consider the new data.

Avastin, also known as bevacizumab, is already approved in the United States as a treatment for colorectal and lung cancers. Genentech sold $2.3 billion worth of Avastin in 2007, all in the United States. An agency advisory committee voted 5 to 4 in December that Avastin should not be approved for breast cancer.