Thirty-one post-menopausal female patients, with locally advanced or disseminated breast cancer were treated with the aromatase inhibitor 4-hydroxyandrostenedione given orally at a dose of 500 mg daily. Twenty-nine patients had assessable disease. Eight patients (28%) had objective evidence of partial response and six remain in remission 7-10 months later. A further four patients (14%) had stabilisation of disease and 11 patients (37%) had progressive disease in spite of treatment. Plasma oestradiol levels were measured throughout therapy in 16 patients and were lowered to 53% +/- 8% of baseline levels within 7 days of commencing 4-hydroxyandrostenedione. With regard to toxicity, one patient developed a transient skin rash and another patient some facial swelling. A further patient developed a transient leucopaenia and treatment was therefore discontinued. Twenty-seven of the 30 evaluable patients (90%) experienced no side effects. These results indicate that oral administration of 4-hydroxyandrostenedione is an acceptable new treatment for post-menopausal women with disseminated breast cancer.