The world's second-biggest drugmaker says its drug, combined with another medicine called letrozole, increased the time patients survived without tumors growing, compared with women just getting letrozole.
Pfizer shares jumped 99 cents, or 3.3 percent, to $31.40 in late-morning trading.
Analysts are closely watching palbociclib, which is for postmenopausal women with certain tumor characteristics found in about 60 percent of patients with advanced breast cancer. It's seen as a potential huge seller.
New York-based Pfizer is now enrolling patients in a late-stage test of palbociclib. Last April, the Food and Drug Administration designated it a breakthrough therapy, a recognition that helps speed up the review needed for approval.