Despite the ongoing problems of undernutrition and infectious disease, obesity and overweight have become a major problem in developing countries, including Bangladesh. This cross-sectional study was undertaken to determine the prevalence of obesity, overweight and underweight among school children aged 6–12 years in Bangladesh. The study was conducted from June 2012 to May 2013 and the study sample comprised 1768 children (980 boys; 788 girls) from eight purposively selected schools in different areas of Dhaka city. Students were interviewed about their diet and physical activity, and anthropometric measurements were made, including height, weight, mid-upper-arm circumference (MUAC), waist circumference, hip circumference and body mass index (BMI). Undernutrition, overweight and obesity were defined using internationally accepted BMI cut-off points. Mean height, weight, BMI, MUAC, waist circumference and hip circumference values were found to be higher in boys than in girls, except at age 12 when these were found to be significantly higher in girls than in boys (p<0.05). The mean prevalence of overweight was 10.0% (boys 10.2%; girls 9.8%), and that of obesity 5.0% (boys 4.3%; girls 5.8%). The prevalence of underweight was 16.3% in boys and 12.7% in girls. The prevalence of underweight was significantly higher in poor than in rich children (22.1% vs 11.2%) and that of obesity was higher in rich than in poor children (9.9% vs 1.3%; p<0.001). A family history of obesity and hypertension emerged as a significant predictor of developing overweight and obesity (p<0.001). The data suggest that underweight and obesity co-exist in urban areas of Bangladesh, posing a challenge for the nutritional health of Bangladeshi children.