We assessed the effect of the method of feeding on respiratory and gastrointestinal illnesses during the first 6 months of life among 776 infants born in New Brunswick, Canada. During a 1-year period, these infants were drawn from the offspring of a population of primiparous women in the province who, after at least 36 weeks of pregnancy, gave birth to one normal infant weighing 2500 gm or more. Data were collected by means of a self-administered standardized questionnaire mailed to every mother a week before her infant reached 6 months of age. The crude incidence density ratio (IDR) revealed a protective effect of breast-feeding on respiratory illnesses (IDR = 0.66; 95% confidence interval (CI), 0.52 to 0.83), on gastrointestinal illnesses (IDR = 0.53; 95% CI, 0.27 to 1.04) and on all illnesses (IDR = 0.67; 95% CI, 0.54 to 0.82). The protective effect of breast-feeding on respiratory illnesses persisted even after adjustment for age of the infant, socioeconomic class, maternal age, and cigarette consumption (adjusted IDR = 0.78; 95% CI, 0.61 to 1.00). Moreover, if we distinguished ear infection from other respiratory illnesses, we observed a separate protective effect for these two types of events. The results of this retrospective cohort study suggest a protective effect of breast-feeding in our population during the first 6 months of life.