The objective of this study was to assess the association between infant diarrhea and ingestion of water containing elevated sulfate levels. The authors identified 274 mothers of infants born in 19 South Dakota counties with high water sulfate concentrations. Demographic information and seven-day- recall dietary and health data were obtained by telephone interviews. Sulfate in drinking water was measured from samples submitted by the participants. Logistic regression was used to estimate the risk for diarrhea (>/=3 loose stools in 24 hours). Questionnaires were completed for 274 households: 69% drank municipal water and 54% reported using the water in the infants' diets. Thirty-nine infants developed diarrhea. Of the 170 households that submitted water samples, 141 (83%) were using the water in the infants' diets. The median sulfate level of the water samples was 264 mg/L. Twenty-five of the infants developed diarrhea. Average infant daily sulfate intake was not significantly associated with an increased diarrhea rate. There was no significant association between sulfate ingestion and the incidence of diarrhea for the range of sulfate levels studied. There was no evidence of a dose-response or threshold effect. However, because of the small number of the most highly exposed infants, the possibility of such an association should be further evaluated.