Underweight represents depleted body fat or lean tissue store and reflects both acute and chronic under nutrition. Under nutrition during the school years can retard child's physical and cognitive development leading to impaired learning abilities. The purpose of the present study was to determine the relationship between underweight and academic performance in middle school children. A cross sectional study was carried out in three co educational schools of Allahabad city-Maharishi Patanjali Vidya Mandir, Tagore Public School and YMCA Centenary School and College. A total of 909 school children from 6th to 8th standards aged between 11 to 13 years were studied. Weight and height of children were measured and Body Index Mass (BMI) was calculated. Children with Body Mass Index below 5th percentile were considered as underweight. 34 Identified underweight children as cases and 34 identified normal weight children as controls were selected. While selecting controls, it was kept in mind to maintain the similarity of age, gender and class between case and control. Cumulative Grade Point Average (CGPA), Math, English and Science grade were recorded from the school register to evaluate children's academic achievement. Chi square test was used for statistical analysis. In the study overall prevalence of overweight, obesity and underweight was 20.2%, 9.0% and 3.8% respectively among middle school children. The proportion of overweight and obesity was found higher among boys (21.9% and 10.9%) as compared to girls (17.6% and 6.2%) whereas the proportion of underweight was found higher among girls (4.8%) than boys (3.1%). On comparison between the underweight and normal weight children's academic achievement results of the study reveal that higher proportion of underweight children (29.4%, 55.9%, 41.2% and 55.9%) obtained poor grade compared to normal weight children (11.7%, 35.3%, 20.6% and 52.9%) in Cumulative Grade Point Average (CGPA), Math, English and Science respectively but the difference was not found significant (p>0.05) statistically. Further studies are required to understand the relation between underweight and academic achievement in children and adolescents.