Total serum testosterone, serum testosterone binding globulin and free androgen index were determined before and during treatment in 14 patients with previously untreated disseminated prostatic cancer. Six patients received estramustine phosphate and six other patients underwent orchiectomy. Two further patients received estramustine phosphate because of tumor progression one and two years after orchiectomy. The result of the study indicates that estramustine phosphate is significantly more effective than orchiectomy in eliciting low levels of free androgens. This complete androgen ablation is produced by a depression of total testosterone and a concomitant increase of total serum testosterone binding globulin which yields a free androgen index an average 4.6 times lower in estramustine phosphate treated patients than in patients who underwent orchiectomy.