There is uncertainty as to whether breast feeding protects against subsequent illnesses; it has been suggested that breast feeding may have some protective effects on the severity of long-term outcome of bronchiolitis and in reducing morbidity. We have assessed the effects of breast feeding in 266 patients and 199 controls, all patients were early wheezers (i.e., under 2 years old). Between these groups we found differences in socioeconomic, environmental, and atopic conditions, but there were no significant differences in the numbers who had been breastfed. However, within the group who had had early wheezing we found that infants who had been breastfed for at least one month subsequently had less severe wheezing. These results suggest that breast feeding may be a protective factor for early wheezing only during the first month of life, and a delaying factor in the following months.