We evaluated gastrointestinal absorption in six consecutive patients with metastatic serotonin-secreting carcinoid tumors. One patient had a consistent defect in fat absorption and two other patients malabsorbed fat during spontaneous or dopamine-induced exacerbation of the carcinoid syndrome. The steatorrhea of the patient with the persistent defect in fat absorption was reduced when tumor serotonin production was reduced by the tryptophan hydroxylase inhibitor parachlorophenylalanine. The six patients had normal hemoglobin levels and the serum concentration of the following urinary constituents was normal in most of the patients: albumin, carotene, 25-hydroxycalciferol, parathyroid hormone, calcitonin, calcium, phosphorous, osteogenous alkaline phosphatase, cholesterol, triglycerides, and serum lipoproteins. The excretion of the following urinary constituents was also normal in most of the patients: creatinine clearance, tubular reabsorption of phosphorous, calcium, D-xylose, cyclic 3'5' monophosphate and hydroxyproline. We conclude that patients with the carcinoid syndrome may have steatorrhea, and that their hyperserotoninemia plays a role in this process.