The use of dual X-ray absorptiometry (DXA) for measurement of bone mineral and body composition in pediatric subjects faces a major technical issue: body size dictates choice of scan mode. However, different scan modes change results in the same subject, thus affecting the accuracy of bone/body composition measurements and especially the capacity to measure changes owing to either growth or intervention. To evaluate the effect of scan mode selections on measurements of bone mineral and body composition, 13 children with weights at the cutoff point between the pediatric large and adult medium scan modes of Lunar DPX or DPXL (Lunar, Madison, WI) with software 3.6 g (35.3 +/- 0.9 kg or 77.7 +/- 2.0 lb) were scanned by both modes. Adult medium mode gave significantly higher results than pediatric large mode for total body fat mass (11.1%), fat% (10.5%), bone mineral content (8.1%), and bone area (1.3%) (p < 0.02). The differences between pediatric large and adult medium modes in fat measurements increased with increasing body mass index ([BMI], kg/m(2)), body surface area ([BSA], m(2)), and trunk size (mm), whereas the differences in bone mineral measurement tended to be greater only with increasing BMI and BSA. None of the differences were correlated to body weight. This study suggests that scan mode selections based on trunk size, BMI, or BSA instead of body weight may improve continuity of bone and body composition measurements by the DXA technique in pediatric subjects.