Twenty-nine patients with metastatic prostate cancer progressing after hormonal therapy (orchiectomy 19, diethylstilbestrol 10) and who had never received cytotoxic therapy were treated with carboplatin. Patients had good clinical performance status (66% PS 0,1) and adequate renal (creatinine less than 2.0 mg/dL) and bone marrow function. The standard dose of carboplatin administered was 400 mg/sq m. Seventeen patients received this dose and 12 either 320 mg/sq m or 250 mg/sq m based on reduced renal function or prior radiation. Five patients had bidimensionally measurable disease: one experienced a partial regression of cervical lymph node metastases of 97 days duration. Twenty-four patients had metastatic disease evaluable by clinical status, bone scan and acid phosphatase. In one patient greater than 50% reduction in number of abnormal areas of bone scan uptake occurred; 3 patients experienced improvement in clinical status; in no patient did an elevated prostate acid phosphatase return to normal. All patients entered on study have progressed and died: median time to progression was 94 days (6 to 625 days); median survival was 297 days (6-1152 days). The primary toxicity of carboplatin was myelosuppression. The median WBC and platelet nadirs after cycle one were 3150/cu mm and 93,000/cu mm, respectively. Dose escalations to grade 2 or greater myelosuppression were mandated. Twenty-six achieved at least grade 2 myelosuppression during carboplatin treatment. We conclude that carboplatin administered at this dose and schedule has no important activity in hormone refractory prostate cancer.