Many studies reported a high prevalence of undernutrition in the under-5 children in Bangladesh. But very few information are available about undernutrition and adiposity among school children and adolescents in Bangladesh. This study addressed the prevalence of undernutrition and obesity among school going children and adolescents. A total of 15 secondary schools were purposively selected from rural, suburban and urban areas. The teachers were detailed about the study protocol. Then the teachers volunteered to register the eligible (age 10 -18y) students for the study. Each student's parent was interviewed for family income. Height (ht), weight (wt), mid-upper arm circumference (MUAC) and blood pressure were taken. Fasting blood samples were collected for fasting plasma glucose, total cholesterol (Chol), triglycerides (TG), high-density lipoproteins (HDL). Body mass index (BMI) was calculated (ht/wt in met. sq) for diagnosis of undernutrition (BMI <18.5), normal weight (BMI 18.5 -22.9) overweight (BMI 23.0 -25.0) and obesity (BMI >25.0). A total of 2151 (m-1063, f-1088) students volunteered the study. Of them, the poor, middle and rich social classes were 25.4, 53.1 and 21.5%, respectively. Overall, the prevalence of underweight, normal, overweight and obesity were 57.4%, 35.0%, 4.9% and 2.7%, respectively. For gender comparison, there has been no significant difference of BMI between boys and girls. By social class, the prevalence of underweight was significantly higher in the poor than in the rich (62.2% v. 43.6%) and obesity was higher in the rich than in the poor (6.1% v. 1.2%) [for both, p<0.001]. Logistic regression showed that the participants from urban (OR 1.51, 95% CI 1.03 -2.22) and the rich (OR 2.03, 95% CI 1.24 -3.33) social class had excess risk for obesity. The risk for undernutrition was found just reverse. Undernutrition was found most prevalent among the rural students and among the poor social class; whereas, prevalence of overweight and obesity appears to be increasing with urbanization and increasing family income. Thus, the study showed a nutrition paradox -adiposity in the midst of many undernourished children and adolescents in Bangladesh. Further study may be undertaken in a large scale to establish diagnostic criteria for age specific nutrition assessment in Bangladesh. A prospective children cohort may help assessing the cut-offs for unhealthy sequels of undernutrition and adiposity.Ibrahim Med. Coll. J. 2012; 6(1): 1-8