To identify environmental influences on infant growth using data from a birth cohort established in 1921. A longitudinal cohort study. Aberdeen 1921-22. Five hundred and sixteen individuals (263 boys and 253 girls) born in Aberdeen during 1921. Health visitor assessments ranged from two to 40 (47% received at least 10 visits). No records were available for infants who died. Individuals were grouped as those who did not breast feed, those who breast fed initially but not at 6 months, and those who were continuing to breast feed at 6 months. Rate of weight gain over the 1st year of life. A random effects model was used to identify environmental factors and conditions contributing to rate of weight gain in the 1st year of life. Breast feeding rates were about 80% and 50% at 10 days and 6 months, respectively. Breast fed infants were significantly heavier than bottle fed infants at 28 days but this difference disappeared by 12 months. Significant negative effects on rate of weight gain, independent of initial body weight, were found for overcrowding in family homes and maternal parity, whereas social class had no effect. Studies based on historical cohorts that have controlled socioeconomic variables only in terms of social class (derived from parental occupation) may have been subject to residual confounding. Growth in the 1st year of life is likely to reflect a number of environmental influences, some of which may continue to have effects throughout early life and beyond.