We treated nine patients who had carcinoid tumors of the small intestine, six of whom had the carcinoid syndrome, with daily intramuscular doses of leukocyte interferon--3 X 10(6) U per day for one month and 6 X 10(6) U per day for another two months. Seven patients had previously been treated with streptozocin and fluorouracil, without benefit. Treatment with interferon ameliorated the manifestations of the carcinoid syndrome and led to prompt and continuing decreases in urinary levels of 5-hydroxyindoleacetic acid and serum levels of human chorionic gonadotropin subunits and pancreatic polypeptide in all six patients with liver metastases, but it had no clear effect in two of three patients with only lymph-node involvement. After the treatment period, five of the six responders had relapses in clinical manifestations and increases in hormone levels. We conclude that interferon is of benefit in treating metastatic small intestinal carcinoid tumors in patients with the carcinoid syndrome.