We followed total prostate and prostatic tumor volumes in patients who received combination endocrine therapy with the antiandrogen flutamide and the LHRH agonist [D-Trp6,des-Gly-NH2(10)]LHRH ethylamide. Twenty-three men with proved prostatic adenocarcinoma (Stages B1 to D2) were subjected to a transrectal ultrasound (TRUS) study before and after a three-month period of combination antihormonal therapy. A total prostatic volume reduction ranging from 17 percent to 70 percent (median 47%, p less than 0.0001) was observed. An even greater effect was observed on tumor volume which was reduced by 20 percent to 91 percent (median 81%, p less than 0.01). After treatment, the original suspicious zone became nonvisible in 4 cases. The TRUS measurements were confirmed by direct examination of the radical prostatectomy specimen in 7 cases. TRUS is thus a precise, sensitive, and valid method for evaluating the effect of combined antihormonal therapy on normal and tumoral prostatic tissues. These data indicate that combination therapy induces a rapid and marked reduction in glandular and tumoral prostatic volume which could well improve the success of radical prostatectomy and increase the changes of cure of localized prostatic cancer.