Ketoconazole is an orally administered broad-spectrum antifungal agent that acts through the inhibition of the steroid synthetic pathways. At high doses in humans ketoconazole can lower rapidly serum testosterone and maintain it in the castrate range with frequent administration. This property suggested that ketoconazole might be useful in the treatment of prostatic cancer. We report a case of prostatic cancer in which ketoconazole resulted in rapid and sustained reduction in serum androgens as well as rapid induction of a clinical remission. Ketoconazole may be a valuable agent in the treatment of prostatic cancer.