Is the treatment of metastatic carcinoid tumor with interferon not as successful as suggested? Eight patients with metastatic carcinoid tumor, seven of whom had symptoms of the carcinoid syndrome, were treated with either human leukocyte interferon (seven patients) or recombinant alpha-interferon (IFN alpha-2b) (one patient) at doses of 4.5 to 21 x 10(6) IU weekly for 1 to 21 (mean, 8.5) months. Tumor regression on computed tomography (CT) scan was found in one patient, the CT findings remained unchanged in three, and the tumor progressed in four patients. A clearcut and continuing decrease in urinary levels of 5-hydroxyindoleacetic acid (5-HIAA) was observed in one patient and a transient one in four patients. The symptoms improved in only two of seven patients. Four patients had leukopenia develop, which was circumvented by reducing the dose. The authors conclude that interferon therapy of the carcinoid tumor is not as successful as has been suggested in previous reports. 