Between April, 1984 and March, 1985 in 14 centers, 33 patients with prostatic cancer of our 14 centers were randomized to subcutaneously receive either 125, 250, or 500 micrograms/day of ICI 118,630, an LH-RH analogue, for 12 weeks, and the clinical efficacy, safety and endocrinal effects of the drug by dose were examined. An objective partial response was obtained in 44.4% of the 125 micrograms-treated group, in 50.0% of the 250 micrograms-treated group, and 42.9% of the 500 micrograms-treated group, showing no significant difference between the groups. General subjective response rates in these three groups were respectively 75.0%, 62.5%, and 85.7%, not differing from each other significantly. No significant difference was observed in endocrinal effect which was seen in 58.3%, 66.7%, and 100% of 125 micrograms, 250 micrograms, and 500 micrograms groups, respectively. Medical castration was attained in 4.1 +/- 2.0 weeks on average. Adverse reactions observed included fever up to 37 degrees C in 1 of the 13 (7.3%) patients treated with 125 micrograms, and hot feeling at the injection site and flare in 1 of the 8 (12.5%) patients treated with 500 micrograms, which were all mild. These patients responded to further ICI 118,630 therapy without requiring any treatment for the adverse reactions.