To evaluate the treatment of advanced islet-cell carcinoma, we randomly assigned 84 patients to streptozocin alone or streptozocin plus fluorouracil. Each regimen was given in five-day courses. The most frequent toxic effects were nausea and vomiting, mild and reversible renal toxicity, and bone-marrow depression with the combination regimen. The combination had advantages over streptozocin alone in overall rate of response (63 vs. 36 per cent) and in rates of complete response (33 vs. 12 per cent). There was no evidence of a preferential response among types of functional tumors. Objective responses were generally of long duration (median, 17 months) and of substantive clinical benefit. Treatment with the combination also yielded a survival advantage over treatment with streptozocin alone (medians, 26 and 16 1/2 months), but this difference is not statistically significant. In spite of gastrointestinal side effects, streptozocin combined with fluorouracil appears to be a valuable treatment for advanced islet-cell carcinoma.