Background/Objective: To determine the body composition of adolescents with spinal cord injury (SCI) and to assess whether established cutoff values for obesity determined by body mass index (BMI) are valid for this population.Methods: Sixty patients, aged 10-21 years, with traumatic SCI (50 with paraplegia and 10 with tetraplegia) were compared with 60 gender-, age-, and BMI-matched controls (CTRL). Dual-energy x-ray absorptiometry was used to estimate regional and total bone mineral content, lean tissue mass, fat tissue mass, and body fat percentage. BMI was calculated from measured weight and stature (kg/m 2 ).Results: Total percent body fat was significantly higher in the paraplegia group (31.4% 6 1.2%; mean 6 SE) than in the tetraplegia and CTRL groups (25.7% 6 2.7% and 22.9% 6 1.1%, respectively). This change in percent total body fat was associated with a reduction of lean tissue mass in the paraplegia (37.6 6 1.1 kg; mean 6 SE) and tetraplegia (32.8 6 2.5 kg) subjects as compared to the CTRL group (46.2 6 1.0 kg; P , 0.001). Total fat mass was significantly greater in the paraplegia group (19.3 6 1.3 kg) than the CTRL and tetraplegia groups (14.9 6 1.2 kg and 11.7 6 3.0 kg, respectively). Regional measurements revealed that the greatest reduction of lean tissue mass in the SCI subjects occurred in the lower extremities, followed by the trunk. As a result of these changes in body composition, the optimal BMI for classifying obesity (trunk fat percent .30 in males and .35 in females) in subjects with SCI was 19 kg/m 2 as compared to 25 kg/m 2 in able-bodied subjects.Conclusions: Patients aged 10 to 21 years with SCI have significantly decreased lean tissue mass and bone mineral content, and increased fat mass. As a result, traditional BMI cutoff criteria significantly underestimate obesity in this population. New clinically applicable criteria to define obesity should be established for SCI children and adolescents with SCI.J Spinal Cord Med. 2007;30:S97-S104