Thirty-two patients with metastatic carcinoma of the prostate refractory to endocrine therapy were entered on trial. No patient entered in the study had prior chemotherapy. Patients were treated with 5-fluorouracil given at a dosage of 4 gm/m2 over a 24-hour period every 2 weeks. Of the 27 patients evaluable for response, there were no complete or partial remissions, but 9 (33%) had a stable disease. The 95% confidence interval for complete and partial response in this series (0 of 27 patients) is 0.0-12%. Myelosuppression and gastrointestinal toxicity was moderate. Two patients, however, experienced major but completely reversible neurotoxicity, including 1 with cerebellar ataxia and 1 with memory loss and stroke-like symptoms. These data indicate that high-dose fluorouracil used in this dosage and schedule is ineffective in the therapy of advanced carcinoma of the prostate.