Objective: To assess the impact of breast-feeding on childhood overweight/obesity in an Eastern European socialist society with relatively homogeneous lifestyles. Study design: Cross-sectional survey data collected in 1991 on 33,768 school-children aged 6 to 14 years in the Czech Republic were analyzed by using multiple logistic regression analyses (main outcome body mass index [BMI] >90th percentile [overweight] and BMI >97th percentile [obesity]). Results: Overall prevalence of overweight (obesity) was lower in breast-fed children: ever breast-fed (9.3%; 95% CI, 8.9-9.6 [3.2%; 95% CI, 3.0-3.4]) compared with never breast-fed (12.4%; 95% CI, 11.3-13.6 [4.4%; 95% CI, 3.7-5.2]). The effect of breast-feeding on overweight/obesity did not diminish with age in children 6 to 14 years old and could not be explained by parental education, parental obesity, maternal smoking, high birth weight, watching television, number of siblings, and physical activity. Adjusted odds ratios for breast-feeding were for overweight 0.80 (95% CI, 0.71-0.90) and for obesity 0.80 (95% CI, 0.66-0.96). Conclusions: A reduced prevalence of overweight/obesity was associated with breast-feeding in a setting where socioeconomic status was homogeneous. This suggests that the effect of breast-feeding on the prevalence of obesity is not confounded by socioeconomic status. (J Pediatr 2002;141:764-9)