Endogenous androgen levels of prostatic tissues in patients with benign prostatic hypertrophy (BPH) and untreated prostatic cancer (PC) were determined by radioimmunoassay using sephadex LH-20 column chromatography. By this assay procedure, it was possible to measure tissue androgen levels with only 30 approximately 40 mg weight of tissues. In BPH (n=20) the concentrations of steroids (mean +/- standard deviation, ng/g. tissue weight) were 5 alpha-androstane 3 alpha, 17 beta diol (A-diol), 2.31 +/- 1.29; dihydrotestosterone (DHT), 4.97 +/- 2.17; testosterone (T), 0.62 +/- 0.33. In PC (n=17) the concentrations were 5.51 +/- 3.23; 3.94 +/- 3.16; and 0.93 +/- 0.38; respectively. The concentrations of DHT were raised and the levels of A-diol were reduced significantly (P less than 0.01) in BPH compared with PC. Tissue DHT levels of PC were variable, but they could be classified in to two groups. One was below 2 ng levels (n=8) similar to those of non-androgen target tissues and the other was over 2 ng (n=19). The relationships between histological differentiation and tissue DHT levels in prostatic cancer were examined. In poorly differentiated cancer (n=4), DHT levels were low in all cases, furthermore even in well differentiated ones there were 4 cases with levels below 2 ng. Thus, the measurement of endogenous DHT levels of prostatic tissues was considered to be valuable for the judgment of androgen dependency in prostatic cancer.