Metastatic Wilms' tumor was temporarily controlled by vincristine (leurocristine) sulfate in 9 of 13 children. The responses to the drug were prompt, becoming apparent within 3 weeks in 75% of the patients. The duration of the responses was relatively short, however, being less than 2 months in half the group. No cross-resistance in antitumor effect with previously administered chemotherapy or radiotherapy was observed. The vincristine was administered in "priming" doses for 5 days, and thereafter in weekly maintenance doses. Although some of the patients had two or more toxic manifestations, i.e., alopecia, nausea, vomiting, increased irritability, pain in the abdomen and in the jaws, disturbances of gait, and hematuria, only two patients required a temporary alteration of the dosage regimen. No deleterious effects upon the body weight or blood count of any of the patients were detected. The potential usefulness of vincristine sulfate as a palliative treatment for patients with Wilms' tumor is discussed.