The impact of breast-feeding on overweight and obesity in children at school entry was assessed in a cross sectional study in Bavaria in 1997. The school entry health examination enrolled 134,577 children. Data on early feeding were collected in two rural districts (eligible population n=13,345). The analyses were confined to 5 or 6 year old children with German nationality. The main outcome measures were overweight (BMI>90th percentile for all German children seen at the 1997 school entry health examination in Bavaria) and obesity (BMI>97th percentile). Information on breast-feeding was available for 9206 children of whom 56% had been breast-fed for any length of time. In non breast-fed children the upper tail of the BMI distribution was enlarged as compared to the breast-fed children whereas the median was almost identical. The prevalence of obesity in children who had never been breast-fed was 4.5% as compared to 2.8% in ever breast-fed children. A clear dose response effect for the duration of breast-feeding on the prevalence of obesity was found: 3.8%, 2.3%, 1.7% and 0.8% for exclusive breast-feeding for up to 2, 3 to 5, 6 to 12 and more than 12 months, respectively. The results for overweight were very similar. The protective effect of beast feeding on overweight and obesity could not be explained by differences in social class or lifestyle. The adjusted odds ratios of breast-feeding for any length of time was 0.71 (95% CI 0.56-0.90) for obesity and 0.77 (95%CI 0.66-0.88) for overweight. This data set did not allow to adjust for maternal weight, an important risk factor for obesity in children. Maternal overweight, however, could not explain the effect of breastfeeding on overweight and obesity in a similar study. The reduction in the risk for overweight and obesity is therefore more likely to be related to the properties of human milk than to factors associated with breast-feeding. The potential relevance of different components of human milk for the observed reduction in the risk for overweight and obesity is discussed. The preventive effect of breast-feeding on overweight and obesity is an important additional argument for the promotion of breast-feeding in industrialised countries.