ABSTRACT It remains unclear whether breast-feeding protects infants against acute respiratory infection (ARI) To determine if breast-feeding protects against ARI as it does against diarrhea, 170 healthy newborns were followed for 6 mo. Feeding mode, incidence and duration of ARI and diarrhea were recorded biweekly. Infants were classified as fully or partially breast-fed, or formula-ted. Incidence and prevalence were computed monthly. The effects of duration of breast-feeding and potential confounders were analyzed by multiple and logistic regression analyses. Incidence and prevalence of ARI were significantly lower in fully breast-fed infants than in formula-fed infants from birth up to 4 mo, as was the mean duration of individual episodes (5.1 ± 3.5 vs. 6.4 ± 3.6 d,respectively). Incidence of ARI was negatively associated with duration of breast-feeding and positively associated with the presence of siblings (P < 0.05). The prevalence of ARI was associated only with the duration of breast-feeding (P < 0.05). Infants that were never breast-ted and that had one or more siblings were more likely to have an episode of ARI than those fully breast-fed for at least 1 mo. Incidence, prevalence, and duration of individual episodes of diarrhea were also lower in breast-fed infants. Incidence (r = —0.17, P < 0.02) and prevalence (r = -O.l9, P < 0.008) were negatively associated with duration of full breast-feeding. Introduction of solid food was not associated with further episodes of diarrhea. The present results demonstrate protection against ARI as a result of breast-feeding similar to that for diarrhea, i.e., lower incidence and percentage of days ill, and episodes of shorter duration. J. Nutr. 127: 436-443, 199 ?.