BACKGROUND AND OBJECTIVE Breastfeeding is considered to be a protective factor against obesity in healthy children. But previous studies about the effect of breastfeeding in the offspring of diabetic mothers have provided inconsistent results. It was the aim of this study to assess the effect of breastfeeding on the risk of becoming overweight at two years of age in children of these mothers. METHODS In a prospective cohort study data on exclusive or partial breastfeeding habits, were obtained from questionnaires given to 816 mothers, aged between 17 and 43 years, with type 1 diabetes. Weight and height of their children was obtained from their pediatric records. Children with a BMI > or = 90th percentile were classified as being overweight. RESULTS 77.9 % of all mothers started breastfeeding after the child's birth. Six months later 33.1 % were still breast-feeding. 14.5 % of all children were overweight at two years of age. After adjusting for smoking in pregnancy and the child's gender and birth weight, there was a risk reduction for overweight of 60 % (OR=0.405; 95%-CI: 0.211-0.779) for breastfeeding duration of 12 to 25 weeks. A similar positive association was found for predominant breastfeeding for a duration of at least four months (OR=0.500; 95%-CI: 0.282-0.887). Protective effects were already present when the duration of breastfeeding was four weeks or more. CONCLUSIONS Mothers with type 1 diabetes breastfed less and for a shorter duration than those in the general population. Breastfeeding duration of at least four weeks was associated with a reduced risk for overweight at two years of age. Special needs regarding diabetes management have to be taken into account.