During the past 5 years, 5-fluorouracil (5-FU) was used to treat 1,091 patients with from 1 to 54 courses. An attempt was made to treat each patient to slight toxicity with each course of therapy given at monthly intervals except poor-risk patients who either were not treated or given a poor-risk schedule. Of 428 patients with measurable lesions treated with 5-FU, 91 met all our criteria of improvement for an average duration of 9 months before reactivation of the disease occurred. The lesions found most responsive to the drug were cancer of the breast, colon or rectum, stomach, cervix, ovary, and malignant hepatoma. Squamous-cell carcinoma of the head and neck, carcinoma of the lung, hypernephroma, and malignant melanoma were unresponsive.