We used an octapeptide analogue of somatostatin, SMS 201-995, in dosages ranging from 150 to 450 micrograms/d administered subcutaneously in three daily doses for 1 to 16 months, to treat 22 patients with advanced malignant islet cell carcinomas. Of the 22 patients, there were 9 with gastrinomas; 3 with glucagonomas; 4 with insulinomas; 1 with ectopic production of parathyroid hormone; and 3 with mixed syndromes. The only biochemical marker in 1 patient was pancreatic polypeptide, and 1 patient had no demonstrable peptide production from the tumor. In 14 patients, dramatic decreases in the levels of circulating peptides (insulin, vasoactive intestinal polypeptide, gastrin, and glucagon) have been accompanied by major alleviations of symptoms. Steatorrhea appears to be the most significant toxicity. This analogue of somatostatin may be appropriate for use as early therapy in patients who have symptoms from syndromes related to islet cell carcinomas but in whom there is no immediate threat from tumor progression.