Background: Upper arm composition is a reflection of body protein and calorie reserves. However, there is a paucity of data on upper arm composition of children from African countries, including Nigeria. This study aimed to determine the composition of upper arm and nutritional status of school children in Abeokuta, Nigeria and to compare with international reference standards. The sensitivity and specifi city of upper arm muscle area by height (UAMAH) as a nutritional assessment tool was also determined. Methods: Five hundred and seventy children aged 5 to 19 years were selected from seven schools using multistage random sampling. Weight, height, mid-upper arm circumference (MUAC) and triceps skin fold thickness (TSF) were measured. Body mass index, upper arm muscle area (UAMA), upper arm fat area (UAFA), fat percentage and UAMAH were derived. Results: The TSF, UAFA and fat percentage were significantly higher in females than males at each age group. MUAC and UAMA were significantly higher in female children aged 10-14 years, whereas UAMA was significantly higher in male children aged 15-19 years. UAMA and UAFA of the children were lower than those of Americans but similar to those of Zimbabweans, and higher than those of Indians. The sensitivity and specifi city of UAMAH for detecting wasting were 80.8% and 63.9%, respectively, whereas the corresponding values for stunting were 32.2% and 58.2%, respectively. Conclusions: The school children studied have a combination of poor calorie and protein reserve. UAMAH may be a valuable tool for complete evaluation of the nutritional status of school children.