Testosterone (T) and dihydrotestosterone (DHT) were measured in the seminal plasma of men with varicoceles, azoospermic vasectomized men and men with normal seminal parameters. Testosterone levels were not different significantly in these groups of men. DHT concentrations and the DHT/T ratio were significantly lower than normal in men with varicoceles and sperm densities below 30 million/ml and the vasectomized men, but not different from normal in the men with varicoceles whose sperm densities were greater than 30 million/ml. In men whose sperm density and/or motility improved after varicocelectomy, there was an increase in seminal plasma DHT levels and the DHT/T ratio. When seminal parameters were not improved by varicocelectomy, there was no change in DHT levels or in the DHT/T ratio. It is concluded that men with varicoceles and sperm densities below 30 million/ml have a deficiency in the epididymal 5 alpha-reduction of testosterone.