Vitamin A was administered to randomly allocated patients in a group of 100 patients with metastatic breast carcinoma treated by chemotherapy. The daily doses (given indefinitely) ranged from 350,000 to 500,000 IU according to body weight. A significant increase in the complete response rate was observed. When subgroups determined by menopausal status were considered, it was observed that serum retinol levels were only significantly increased in the post-menopausal group on high dose Vitamin A. Response rates, duration of response and projected survival were only significantly increased in this subgroup. The therapeutic and biological implications of these findings are discussed.