A sensitive and reliable radioimmunoassay (RIA) for urinary unconjugated 5 alpha-androstane-3 alpha, 17 beta-diol is described. The mean overall recovery of unconjugated 5 alpha-androstane-3 alpha, 17 beta-diol was found to be 57.4%. The sensitivity of the assay was 79 fmol per assay tube and the intra and inter-assay variations ranged between 7.2% and 11.4%. The mean +/- SEM for the concentration of this androgen in the urine of normal men was 339.6 +/- 66.8 nmol/24 h. The corresponding values for patients with benign prostatic hypertrophy (BPH) and carcinoma of the prostate (Ca) were 297.8 +/- 44.7 and 1592.1 +/- 622.7 respectively. The mean value for Ca patients was significantly higher than either BPH (p less than 0.05) or normal subject (p less than 0.02), suggesting a differential urinary excretion pattern for unconjugated 5 alpha-androstane-3 alpha, 17 beta-diol between BPH and Ca patients. It is concluded that the combined measurement of this androgen in the plasma and urine provides a more accurate assessment of the profile of this hormone than a single plasma estimation.