The respective roles of reduced dietary intake and malabsorption in the pathogenesis of weight loss in persons with chronic tropical sprue have been evaluated . Dietary intake was found to be significantly (P less than 0.001) less in a group of 45 patients with tropical sprue, all of whom had anorexia due to deficiency of folate and/or vitamin B12, than in a group of 51 healthy Puerto Ricans. Weight loss was equally prominent in those patients with tropical sprue who had normal absorption of fat and protein as in those who had excessive fecal loss and reduced absorption of these nutrients. Treatment of five sprue patients with folic acid or vitamin B12 for 2 weeks resulted in improved appetite and increased in dietary intake with weight gain in the absence of significant improvement in intestinal absorption. Treatment with oral tetracycline for a similar period of time in five other patients was not associated with vitamin repletion, return of appetite or weight gain. These observations indicate that reduced dietary intake resulting from anorexia caused by vitamin deficiency is a significant, and sometimes the most important, factor in the pathogenesis of weight loss in persons with chronic tropical sprue.