A 39 year old male patient was treated with daily 5 X 10(6) IU interferon alpha 2b s.c. for 22 months because of advanced carcinoid tumor. Objective response was achieved with greater than 50% reduction of 5-HIAA-excretion. Multiple metastases (liver, lung, bones, thyroid gland) were not progressing in size. An unintentional omission of treatment resulted in a rapid biochemical and morphological tumor progression. Reversibility was achieved with reinstitution of effective interferon therapy. Thus, interferon therapy of advanced carcinoid tumor is able to induce a long-term objective response. It seems to be superior to conventional chemotherapy.