In a randomized controlled study, 20 patients with malignant carcinoid tumors were included. Ten patients received streptozocin plus 5-fluorouracil for 6 months and another 10 human leukocyte interferon (IFN). After 6 months of treatment, an objective tumor response was noted in five of the patients treated with IFN (50%) but in none of the patients on chemotherapy. Stable disease was found in five patients (50%) on IFN treatment and four (40%) on chemotherapy. Progressive disease was noted in six of the patients (60%) receiving chemotherapy. A statistical analysis using the chi-square test showed a significantly higher proportion of responders and stable disease in the IFN treated group (P = 0.0039). Furthermore, three of eight patients who had previously received chemotherapy showed later on an objective response to IFN. The objective responses were mainly noted in decreased tumor markers; however, two patients also showed a significant reduction of tumor size. Subjective responses were noted in 72% of patients treated with interferon, but only in 9% of those treated with streptozocin plus 5-fluorouracil. The results indicate that interferon treatment is superior to the combination of streptozocin plus 5-fluorouracil. Considering both the therapeutic effects and adverse reactions, human leukocyte interferon is a promising alternative for treatment of patients with malignant carcinoid tumors.