One hundred twenty-four patients with metastatic breast cancer were treated with 40 mg of megestrol acetate four times daily. Complete responses (CR) or partial responses (PR) were seen in 29 patients (23%). CR, PR, or stable disease (S) was seen in 80 patients (65%). The median duration of response was 22 months for CR and PR and ten months for S. A significantly higher response rate (CR + PR) was seen in estrogen-receptor-positive (ER-positive) patients (26%) and in patients who had not received prior therapy (39%). A significant relationship to response could not be established for menopausal status, progesterone-receptor (PrR) status, dominant site of disease, or prior administration of chemotherapy. Median survival was 66+ months for responders, 35 months for patients with stable disease, and 9 months for nonresponders. These differences are all statistically significant (P less than .001). Toxicity was minimal, and side effects consisted primarily of weight gain, which was seen in 18 patients (14.5%). Megestrol acetate can provide effective palliation in patients with advanced breast cancer.