Increased blood serotonin levels occur in nontropical sprue and in other malabsorption states. This finding and the previous observations of alterations in levels of other indoles in malabsorption diseases indicate extensive disturbance in tryptophane metabolism involving many of the metabolic pathways of this amino acid. It is probable that this metabolic disturbance contributes in a number of ways to the disease process, but precise understanding is lacking. On the basis of the preliminary data obtained in this study of 14 patients, no correlation is seen between the levels of blood serotonin and the degree of steatorrhea. However, the degree of elevation of blood serotonin appears slightly greater in patients with nontropical sprue than in patients with malabsorption due to a variety of other causes.