Background: Bloody diarrhea is associated with morbidity and mortality in infants and children worldwide. Prevention of this condition is one of the major goals of all diarrheal control programs. Objective: To determine if breast feeding protects against bloody diarrhea in children under 18 mo of age from a periurban area of Mexico City. Methods: From April 1988 through December 1991, a cohort of 316 infants was followed prospectively from birth to 18 mo of age. Weekly household information was obtained on the occurrence of diarrhea, presence of blood in stools and feeding intake. Sociodemographic and hygiene characteristics also were obtained. A stool sample was collected during each visit and tested for enteropathogens, regardless of symptom status. Risks were estimated by Cox proportional hazard model. All comparisons were evaluated at P=.05 (2-tailed). Results: There were 385 child-years of follow-up. Annual incidence of bloody diarrhea was 24 episodes/100 child-years. Bloody diarrhea was observed in all age groups with no differences between genders. Incidence of bloody diarrhea was highest during June and August (3.2 episodes/100 child-mo), the warmest months of the year.Campylobacter spp. was the pathogen more frequently isolated (26%). Incidence of bloody diarrhea among breastfed children was lower when compared with children not receiving human milk (21 episodes/100 child-year vs. 27 episodes/100 child-year; RR=0.63, 95%CI=0.39-1.00). This protective effect was not modified when adjusted by age, gender and season. Conclusions: There was a high incidence of bloody diarrhea among Mexican infants. Breast feeding prevented 35% of the episodes of bloody diarrhea in children from this periurban community. Breast feeding should be encouraged and supported by all health care professionals.