OBJECTIVE: To investigate whether early weaning constitutes a risk factor for overweight at preschool age and to identify other factors that affect this association. METHODS: This was a case-control study of 366 children aged 2 to 6 years (176 boys and 190 girls) from three cities. The case group comprised overweight children, as defined by body mass index (BMI) for age greater than or equal to the 85th percentile. The main exposure analyzed was early weaning (exclusive or predominant breastfeeding for less than four months). RESULTS: Early weaning was a significant risk factor for overweight in univariate analysis (OR = 1.69; 95% CI: 1.10-2.60; p = 0.02), but not in multivariate analysis (OR = 1.42; 95% CI: 0.86-2.34; p = 0.17). Maternal overweight, birth weight > 3,500 g and sedentarism were the main risk factors for overweight in multivariate analysis. DISCUSSION: In our study, the protective effect of breastfeeding against overweight was only shown in univariate analysis; it did not persist after controlling for other variables. It is possible that breastfeeding has only a small protective role against overweight in comparison with other variables of greater importance. CONCLUSION: Our results suggest that the potential protective effect of breastfeeding against overweight among preschool children is weaker than genetic and other environmental factors.