Thirty-nine breast-fed and 42 bottle-fed infants were followed up from birth over a four-year period. Virus infection was documented by culture and serologic testing, and history and physical examination were recorded for all episodes of respiratory illness. There were no statistically significant differences in rates or distributions of infection with individual viruses or with all viruses over the first three or six months or during the second six months of life in the two groups, nor were there statistically significant differences in rates or distributions of disease of the upper and lower respiratory tract or total respiratory disease, except for decreased disease of the lower respiratory tract in bottle-fed infants in the second six months. There were trends to decreased morbidity in breast-fed infants in the first three and six months and more episodes of pneumonia and bronchiolitis in bottle-fed infants in the first six months (P less than .05) but similar use of medical care by both groups. High cord blood titers to two viruses were not associated with evidence of breast-feeding protection from infection with those two agents. Breast-fed babies do not have fewer respiratory virus infections or illnesses but may experience less severe illness.