4-Hydroxyandrostenedione (CGP32349; 4-OHA) is a clinically effective treatment for advanced postmenopausal breast cancer by both the parenteral and p.o. routes, as a result of its inhibition of aromatase and consequent suppression of plasma estrogen levels. Thirty patients were randomized to treatment with 250 mg 4-OHA orally once, twice, and 4 times daily for 2 weeks and 29 of these plus a further 11 patients were then randomized to treatment with 250 or 500 mg i.m. every 2 weeks to determine the optimal dose for each route according to the suppression of serum estradiol levels. There was no significant difference between the 3 oral doses in their suppression of estradiol levels indicating that the maximum required p.o. dose of 4-OHA is probably 250 mg daily. Suppression by the parenteral dose of 250 mg every 2 weeks was marginally suboptimal but clinical considerations of response and tolerability indicate this as the optimal dose for i.m. injection. 4-OHA had no effect on serum levels of androstenedione, testosterone, or 5 alpha-dihydrotestosterone when given by either route but p.o. treatment with 4 doses of 250 mg daily reduced sex hormone-binding globulin levels by a mean of 34%. Serum levels of estrone as measured by gas chromatography-mass spectrometry were suppressed to approximately 40% of baseline by parenteral treatment. The half-life of 4-OHA p.o. was approximately 3 h, whereas the apparent half-life of injected drug was between 5 and 10 days after a more rapid clearance during the first 4 days after injection.