A new, orally active antiestrogenic steroid, mepitiostane (20 mg/day), was given to 45 patients with advanced breast cancer. The regression rate was 31.1%, or 14 of 45 patients, and a duration of regression of greater than 6 months was obtained in seven patients. Virilizing effects such as hoarseness, hirsutism, and acne were observed relatively often, but there was no evidence of abnormality in the liver function tests or in the serum calcium level.