Patients with poorly differentiated prostatic carcinoma and skeletal metastases were randomized to treatment with 2.6-cis-diphenylhexamethylcyclotetrasiloxane (2.6-cis) and estramustine-17-phosphate (estramustine). Parallel with the clinical study a group of non-randomized patients were treated with 2.6-cis. Cytological regression of the tumor could be registered in half of the estramustine group but not in the 2.6-cis group. There were no drug-related changes in blood chemistry, kidney function tests, hematology or liver enzymes. There was in increase in acid and alkaline phosphatase in both groups but more pronounced in the 2.6-cis group. In both groups follicle-stimulating and luteinizing hormone values were depressed. Testicular and penis atrophy was observed in the 2.6-cis group. Relief of pain and marked improvement of conditions occurred in the majority of the cases in both groups. In general, no tumor regression was observed during administration of 300 mg. 2.6-cis daily for at least 3 months. Some tumor regression was noted during 600 mg. estramustine therapy daily.