The aim of this paper is to investigate whether the lower rate of breastfeeding at 6 months by overweight and obese mothers is primarily due to these women giving up breastfeeding in the first week postpartum using a cross-sectional population survey. The sample is children from the infant cohort (about 12 months of age) of Wave 1 (2004) of the Longitudinal Study of Australian Children for whom breastfeeding and maternal information were available (n = 3075). Definitions used: normal-weight body mass index (BMI, kg/m(2)) 20 to <25, overweight BMI 25 to <30, obese BMI > or =30. Breastfeeding initiation was 95.1% for normal-weight women, 92.8% for overweight women and 87.1% for obese women. At 6 months, 64% of normal-weight women were breastfeeding, compared with 54% of overweight and 44% of obese women. On multivariate analysis, for women who initiated breastfeeding, overweight women had an odds ratio (OR) of 1.52 [95% confidence interval (CI) 1.02, 2.28] and obese women had an OR of 2.54 (95% CI 1.70, 3.79) of stopping breastfeeding by 1 week compared with normal-weight women (adjusted for maternal age, education, smoking, level of socio-economic disadvantage, caesarean birth, admission to special care nursery). For women who breastfed for at least 1 week, overweight women had an adjusted OR of 1.26 (1.04, 1.53) and obese women had an adjusted OR of 1.38 (1.10, 1.73) of ceasing to breastfeed before 6 months, compared with normal-weight women. In conclusion, among overweight/obese women who initiate breastfeeding, higher rates of cessation of breastfeeding in both the immediate postpartum period and in the first 6 months contribute to the shorter duration.