Breastfeeding has long been believed to protect against infection in infants, but protection against respiratory illnesses has not been consistently demonstrated in studies in developed countries. Between 1988 and 1992, the authors assessed the effect of breastfeeding on incidence and duration of respiratory illnesses during the first 6 months of life in a prospective study that actively tracked breastfeeding and respiratory illnesses. A cohort of 1,202 healthy infants, born in Albuquerque, New Mexico, between January 1, 1988 and June 30, 1990, from homes without smokers was enrolled. The daily occurrences of respiratory symptoms and breastfeeding status were reported by the mothers every 2 weeks. Illnesses were classified as lower respiratory illness (LRI) if wheezing or wet cough was reported; the remaining illnesses were classified as upper respiratory. The annualized incidence rates for LRI were 2.8, 2.6, and 2.1 during follow-up time with no, partial, or full breastfeeding, respectively, but the incidence rates for upper respiratory illness and lower respiratory illness combined were similar in the three categories. After adjustment for potential confounding factors, full breastfeeding was associated with a reduction in lower respiratory illness risk (odds ratio=0.81, 95% confidence interval 0.68-0.96). Median duration of all respiratory illnesses was 5 days for the fully breastfed infants during the first 6 months of life compared with a median of 6 days for not breastfed and partially breastfed infants. Multivariate analysis confirmed that breastfeeding significantly reduced the duration of respiratory illness. This pattern of reduced incidence of LRI and shorter duration of all respiratory illnesses suggests that breastfeeding reduces the severity of infant respiratory illnesses during the first 6 months of life.