Chronic nonspecific diarrhea (CNSD) is the most common cause of prolonged diarrhea without failure to thrive. Although it is most commonly seen from ages 6 to 36 months, CNSD may persist until 54 months of age. Forty-four patients with this syndrome had complete dietary histories, and were divided into four groups on the basis of their intakes and responses to its modification. Each of the four groups had significantly less fat in their diet at the time of presentation than did ten non-CNSD patients (P less than .005) presenting similarly. In three of the groups, daily fat consumption was increased, irrespective of the adequacy of their initial intakes. In all 38 patients in these groups, this dietary modification was associated with the resolution of symptoms. The fourth group, with initially normal dietary fat ingestion, did not respond to dietary therapy. The overall success rate of the regimen in this patient population was 82%. Carbohydrate, fiber, and caloric contents of the diets did not appear to play as significant a role as fat intake.