An increased incidence of chronic nonspecific diarrhea has been coincident with popularization of orally administered fluid-electrolyte therapy for management of diarrhea, and led up to postulate than an increase in fluid intake might be related to this increased incidence. Of 105 referred patients, 85 were found to have no clinical or laboratory evidence of malabsorption. Forty of these patients had characteristic features of CNSD: diarrhea for at least three weeks, normal growth, and no evidence of enteric pathogens. An outpatient study evaluated fecal output, dietary energy-protein intake, and nonprotein fluid intake. Patients were separated into two groups whose fluid intakes were highly different: group A, 196 +/- 32 ml/kg/day, and group B, 91 +/- 15 ml/kg/day (P less than 0.001). The nonprotein fluid intake was then reduced to 90 ml/kg/day with no change in diet. Evaluation at two weeks and again at six to eight weeks showed a decrease in stool frequency (from four to ten per day to zero to three per day) and increase in stool consistency in all patients in group A, but no significant change in stool patterns in group B. Our findings suggest a cause-and-effect relationship between excessive fluid intake and some cases of CNSD.