The effect of exclusive breast feeding in the first few months of life on physical growth was studied prospectively in a group of 36 full-term healthy newborns. Growth failure was not observed in any infant until after the third month of life. The average growth curve for the entire group was between the 25th and 50th percentile at the end of the study period. However, altering of growth as judged by weight at or below the tenth percentile of standard for age was seen in three (8%) infants at the age of four months, five (13%) at five months, eight (22%) at six months, nine (25%) at seven months, and twelve (33%) at eight months. Morbidity experience showed a slight but statistically higher frequency of respiratory infections and otitis in those infants who had shown altered growth. Volume of milk intake was similar in the two groups. These observations suggest that a small proportion of exclusively breast-fed infants may not achieve adequate growth. In such infants, consideration should be given to supplementation after 4 months of age.