Because it is well known that prostatic cancer is dependent on testosterone, and that dihydrotestosterone is the active mediator of the androgen action in the prostatic cell, we studied serum concentrations of testosterone and dihydrotestosterone in 84 patients with prostatic cancer, 40 of whom were treated by orchidectomy, and 44 who were not treated. There was a significant correlation between testosterone and dihydrotestosterone concentrations in the untreated group, but no correlation among the patients treated by orchidectomy. The results indicate that the effectiveness of orchidectomy as androgen withdrawal treatment in prostatic cancer should be assessed by measurement of both dihydrotestosterone and testosterone concentrations.