Objectives: We aimed to establish associations of duration of breast-feeding with mean BMI and waist circumference, as well as the likelihood of being overweight/ obese, during early childhood. Design: Cross-sectional, population-based study. Height, weight and waist circumference were measured and BMI calculated. Interviewer-administered questionnaire determined whether the child was ever breast-fed and the duration of breast-feeding. Setting: Sydney, Australia. Subjects: Infants and pre-school children (n 2092) aged 1-6 years were examined in the Sydney Paediatric Eye Disease Study during 2007-2009. Results: Of the children aged 1-6 years, 1270 had been breast-fed compared with 822 who were never breast-fed. After multivariable adjustment, 1-6-year-old children who were ever breast-fed compared with those who were not had significantly lower BMI, 16.7 (SE 0.1) kg/m2 v. 17.1 (SE 0.2) kg/m2 (P=0.01). Decreasing BMI was associated with increasing duration of breast-feeding (Ptrend=0.002). After multivariable adjustment, each month increase in breastfeeding was associated with an average BMI decrease of 0.04 kg/m2 (P=0.002) and 0.03 kg/m2 (P=0.03) among children aged 1-2 years and 3-4 years, respectively. In 1-2-year-old children, each month increase in breast-feeding duration was associated with a 0.06cm decrease in waist circumference (P=0.04). Significant associations were not observed among 5-6-year-old children. Children who were ever breast-fed v. those never breast-fed were less likely to be overweight/obese (multivariable-adjusted OR=0.54; 95% CI 0.36, 0.83). Conclusions: We demonstrated a modest influence of breast-feeding on children's BMI during early childhood, particularly among those aged less than 5 years.