Prostatic blood flow in 19 patients with prostatic adenocarcinoma was measured by the hydrogen gas clearance method before and during endocrine treatment. Prostatic volume was reduced to 70 per cent of the pre-treatment volume by 3 months after the beginning of treatment. Prostatic blood flow was remarkably depressed in patients who had never had any treatment of the prostatic carcinoma (22.2 +/- 8.3 ml. per minute per 100 gm.), while prostatic blood flow increased significantly after endocrine treatment (56.3 +/- 21.8 ml. per minute per 100 gm.). It was likely that prostatic blood flow increased as the prostate volume decreased. Final histology of serial prostatic biopsy specimens after endocrine treatment, revealed distinct deterioration of tumor cells and slight stromal hyperplasia compared to the initial pre-treatment biopsy. The stromal-epithelial ratio, which was calculated by computer-assisted image analysis, was markedly increased after endocrine treatment. Our study indicated that endocrine treatment caused a growth-inhibitory effect that was accompanied by increased blood flow in the prostatic carcinoma.