From April 1984 to May 1986, 129 patients with prostate cancer entered a prospective trial with a new LH-RH agonist, Zoladex. Mean age was 72 years (range of 45-94 years) and, in most cases, patients had metastatic disease, not previously treated by chemotherapy or hormone therapy. Patients received a monthly injection of 3.6 mg. Serum testosterone was lowered into the range of castrate levels after 4 weeks of treatment. In 105 evaluable patients at 3 months, a 65% partial response (PR) rate was observed, with 11% stable and 24% progressive disease. Median time to progression was 37 weeks. Analysis of objective criteria revealed 30% PR for prostate volume and 51% CR-PR for prostatic acid phosphatases. Seventeen percent of lytic metastases had recalcified. One hundred twenty-nine patients were evaluable for toxicity. Endocrinological side effects were common: decrease in libido, 92%; impotence, 86%; hot flushes, 48%; and breast swelling or tenderness, 9%. Nonendocrinologic side effects were rare. The treatment is generally well accepted by patients owing to the convenient depot formulation and to the minor side effects.