Background: Malnutrition is a major public health concern affecting a significant number of school age children influencing their health, growth and development, and school academic performance. Objective: To establish the determinants of under nutrition among school age children between 6-12 years in a low-income urban community. Design: A cross-sectional descriptive study. Setting: Kawangware peri-urban slum, Nairobi, Kenya. Subjects: Three hundred and eighty four school children aged 6-12 years. Results: A total of 4.5% were wasted, 14.9% underweight and 30.2% stunted. The children who were over nine years of age were more underweight (72.4%, p=0.000) and stunted (77.2%, p=0.000) than those below eight years. The girls were more wasted (29.1%, p=0.0 13) than the boys (18.2%), whereas the boys were more stunted (65.7%, p=0.003) than the girls (50.7%). The other variables found to have had significant association with the nutritional status of the children were: monthly household income (p=0.008), food prices (p=0.012), morbidity trends (p=0.045), mode of treatment (p=0.036) and school attendance (p=0.044). Conclusion: The findings of this study show evidently that there is under nutrition among school age children, with stunting being the most prevalent. The Ministry of Education and Ministry of Health therefore need to develop policies which can alleviate under nutrition among school age children. We also recommend that awareness be created among the school age children, parents and teachers, on the dietary requirements of both boys and girls.