An open randomized trial was performed to compare the effect of recombinant interferon-alpha 2a (rIFN-alpha 2a) (group A, n = 12) versus rIFN-alpha 2a in combination with chemotherapy (group B, n = 11) in patients with malignant carcinoid tumors. Both groups received rIFN-alpha 2a at a dose of 3 MU/m2 s.c. three times weekly during the first 6 months. IFN was discontinued every third week in group B, followed by an i.v. injection of 2 g streptozocin and 40 mg/m2 doxorubicin. After 6 months group A showed one complete biochemical response (CR), 9 patients with stable disease (SD) and 2 who progressed (PD). Two patients had a partial reduction (PR) of tumor size, 9 showed SD and one PD. All patients in group B demonstrated SD. Chemotherapy was withdrawn after 6 months and all patients continued with rIFN-alpha 2a at an increased dose of 3 MU/m2 five days/week for a further 6 months. After 12 months 6 patients showed PR, 12 SD and one PD biochemically. Tumor size showed SD in 18 patients and PD in one. One patient died from cardiomyopathy, probably induced by doxorubicin. Antibodies against rIFN-alpha 2a developed in 41% of the patients. In conclusion, we detected no difference in response rates between the two treatment groups. Adverse reactions from the combination were considerable. The frequent development of IFN antibodies might have interfered with the therapeutic results.