Breastfeeding might confer protection against obesity later in life, but the evidence is inconclusive. We tested the hypothesis that breastfeeding is associated with a reduced risk of obesity in a population-based sample of 32200 Scottish children studied at age 39-42 months in 1998 and 1999. Obesity was defined as body-mass index (BMI) at the 95th and 98th percentiles or higher. The prevalence of obesity was significantly lower in breastfed children, and the association persisted after adjustment for socioeconomic status, birthweight, and sex. The adjusted odds ratio for obesity (BMI > or = 98th percentile) was 0.70 (95% CI 0.61-0.80). Our results suggest that breastfeeding is associated with a reduction in childhood obesity risk.