To assess the acceptability and feasibility of whole body air displacement plethysmography in children and to determine its precision and agreement with hydrodensitometry, an appropriate reference method.Age specific two component model equations were used to predict fat mass from body density in 22 children aged 8-12 years and in 10 adults for comparison of methods. Precision for each method was established from duplicate measurements.Plethysmography was accepted more readily than hydrodensitometry (100% v 69% provided duplicate measurements). Precision for fat mass in children was 0.38 kg by plethysmography and 0.68 kg by hydrodensitometry, and results were similar in adults. The mean (SD) fat mass in children was 6.9 kg (4.0) and 6.7 kg (4. 2) by plethysmography and hydrodensitometry, respectively, but 95% limits of agreement between methods were large (-4.1 kg to 3.5 kg fat).Plethysmography was more readily accepted and had better precision than hydrodensitometry. It also provided similar body composition results for the group but not for all individual children.