Ninety patients with advanced prostatic cancer (15 with stage B, 23 with stage C, and 52 with stage D) were randomized to receive 0.9, 1.8, or 3.6 mg, respectively, of Zoladex depot subcutaneous injection every 28 days for 12 weeks. The serum levels of LH, FSH, and testosterone were elevated after the first injection, and followed by a significant decrease. The suppression of testosterone levels in the blood to castrate levels was observed in all patients except two treated with 0.9 mg. Objective response (CR and PR) was seen in 63.6% (0.9 mg), 47.8% (1.8 mg), and 68% (3.6 mg) of patients according to the Japanese Prostatic Group Criteria. Subjective improvement (performance status, analgesic consumption) was also observed in 75-88% of patients but without a statistically significant difference between each dose group. Only minor adverse effects were found during the treatments. The drug was detected dose dependently in the blood by radioimmunoassay. These results suggest that endocrine therapy with Zoladex depot in doses of 3.6 mg subcutaneously every 4 weeks is a useful alternative to surgical castration in patients with prostatic cancer.