We have evaluated 4-hydroxyandrostenedione, a specific inhibitor of aromatase, as treatment for breast cancer in a phase I dose-ranging study and a phase II study of the best-tolerated dose. 168 postmenopausal patients with locally advanced and metastatic breast cancer were treated intramuscularly. 19% of patients attained a complete or partial response but 26% of those who completed at least 4 weeks treatment responded. Side-effects were least in the group receiving 250 mg every 2 weeks. 13% of patients experienced local discomfort due to the injection and 5% had other side-effects. Serum oestradiol fell to 42.4 and 26.5% of baseline at 7 days after the start of treatment with the 250 mg and 500 mg dose, respectively. We conclude that 4-hydroxyandrostenedione at 250 mg every 2 weeks is a safe and effective form of treatment for postmenopausal patients with metastatic breast cancer.