OBJECTIVE: To evaluate the impact of breast feeding on the number of episodes of diarrhea and nutritional status in infants. MATERIALS AND METHODS: From January to April 1993, we enrolled in a case-control study 102 children with acute diarrheal disease (ADD) and 37 healthy children who were <6 months of age, at the Oral Rehydration Unit (ORU) of Children's Health Institute (CHI) in Lima, Perú. We assessed duration of breast feeding, number of episodes of diarrhea, nutritional status, socio-economic status, hygienic behaviour, and maternal age. We used linear regression analysis to determine whether duration of breastfeeding correlated with episodes of diarrhea. RESULTS: 44% of patients with ADD, were breastfed until 1 month of age, 29% until 3 months, and only 27% until 6 months. In contrast, 95% of the healthy controls were breastfed for 6 months. Among infants with ADD, 20% showed mild, 19% moderate, and 40% showed severe malnutrition. Children with shorter breastfeeding had more episodes of diarrhea and were more likely to be malnourished (p=0.01). There was an inverse correlation between the number of episodes of diarrhea and duration of breast-feeding (r=0.46). CONCLUSION: Exclusive breast-feeding for a minimum of four months significantly reduces the number of episodes of diarrhea and malnutrition in children less than 6 months of age.