Twenty two patients with advanced carcinoma of the prostate have been treated for up to 3 months with the slow-release (depot) formulation of the luteinizing hormone-releasing hormone (LHRH) agonist ICI 118630. Patients were randomized to receive one of three different doses of ICI 118630 of 0.9, 1.8 or 3.6 mg. The depot preparation was injected subcutaneously every 4 weeks. At the highest dose, the concentration of testosterone in serum was significantly reduced to castrate values after 2-3 weeks of therapy. The smaller doses of ICI 118630 (1.8 or 0.9 mg every 4 weeks) similarly reduced serum testosterone concentrations although, at the lowest dose, testosterone values were not suppressed in all patients during the first month. Hormonal changes were accompanied by subjective clinical improvement in symptomatic patients and there were no significant side effects. The data clearly demonstrate the considerable therapeutic potential of ICI 118630 in the depot formulation for the treatment of advanced carcinoma of the prostate.