Zoladex, a sustained-release luteinizing hormone--releasing hormone (LHRH) analogue administered by subcutaneous injection every 28 days, was evaluated at three dose levels in 46 men with untreated advanced prostate cancer. All three Zoladex doses yielded similar endocrinologic effects. After initial transient increases in serum luteinizing hormone, follicle-stimulating hormone, and testosterone concentrations, serum testosterone was suppressed uniformly to castration levels within 22 days. At a median follow-up of 41 weeks, Zoladex had maintained persistent suppression of serum testosterone. Measurements of serum Zoladex levels indicated that release of the drug from the injected depot was sustained over a period of 1 month and that there was no drug accumulation as evaluated over an initial 3-month period. No antibodies to Zoladex were detected. Tumor regression rates and side effects with Zoladex therapy were similar to those reported with daily injections of subcutaneous LHRH therapy. Signs and symptoms consistent with a brief tumor flare after the first injection of the LHRH analogue were noted in eight (17%) of the study entrants. Spinal cord compression was observed in two patients within 1 week from the onset of therapy. Zoladex is considered to be an effective, sustained-release LHRH analogue for the treatment of patients with prostate cancer.