Ten breast-fed infants, each paired with a formula-fed infant, were evaluated prospectively from birth to the age of 112 days to identify respiratory and gastrointestinal (GI) illnesses and microorganisms associated with respiratory tract infections. The infants were matched by age and sex. More GI illnesses occurred among the formula-fed infants. Although the breast-fed infants had less exposure to tobacco smoke in the home and less exposure to other children in day-care settings, there was no suggestion of a decrease in the frequency of respiratory illness during the first four months.