Admixture of androgen-sensitive elements from normal or hyperplastic prostatic tissue interferes with biochemical studies of prostate cancer in its primary site. Heterogeneity of cancer tissues, varying in stromal and epithelial elements, also complicates interpretation of data relating to androgen metabolism. Accordingly, we have compared metastatic deposits composed of epithelial cancer cells to the primary biopsies of 4 patients in respect to uptake of 3H-testosterone and its conversion to 5-alpha-dihydrotestosterone during in vitro incubation. 3H-testosterone uptake was similar for both tissue sites but 3H-dihydrotestosterone formation was reduced by 76% in the metastases compared to primary tissues. This group was not large enough to show statistical significance, whereas a total of 11 such primary studies compared to 6 metastatic specimens was significant. When either primary or secondary tissue results were compared to 12 cases of benign prostatic hyperplasia similarly studied the differences were highly significant. These results demonstrate a major impairment in the formation of dihydrotestosterone by metastatic prostatic cancer and a similar but less evident alteration in the primary site. This abnormality in testosterone metabolism is of major importance in the attempt to obtain effective hormonal control of human prostatic cancer.