As part of an extensive search of synergistic combinations of agents that demonstrate cytotoxic activity in tissue culture against prostate cancer cell lines, we have identified estramustine phosphate and docetaxel (Taxotere; Rhône-Poulenc Rorer, Collegeville, PA) as active. Patients with hormone-refractory metastatic prostate cancer with good performance status were entered onto a phase I trial of this combination. Estramustine phosphate was given on a daily oral dosing schedule of 14 mg/kg; docetaxel was administered intravenously over 1 hour every 3 weeks. Prophylactic corticosteroids were administered to minimize taxane side effects. Four dose levels of docetaxel were studied: 40, 60, 70, and 80 mg/m2. Actual weight was used for dosing calculations. An intermittent docetaxel dose of 70 mg/m2 with estramustine phosphate at 12 mg/kg was determined to be a phase II dose and to allow repetitive dosing. Eight additional patients were entered at this dose level of docetaxel. In all patients, the limiting side effects were fatigue and leukopenia. Thromboembolic events also were seen in 16% of patients. In the first 17 patients treated in the phase I aspect of the study, the biochemical response rate was 82%, demonstrating greater than a 50% decline in prostate-specific antigen. Twenty-four percent had normalization of prostate-specific antigen. These early studies indicate that the combination of estramustine and docetaxel has activity in this population of patients.