Sleeve gastrectomy (SG), the most common metabolic and bariatric surgery in adolescents, is associated with bone loss. Marrow adipose tissue (MAT) is a dynamic endocrine organ that responds to changes in nutrition and might serve as a novel biomarker for bone health. Two types of MAT have been described which differ in anatomic location, proximal regulated MAT vs distal constitutive MAT.To determine the effects of SG on volumetric BMD (vBMD) and MAT in adolescents with obesity. We hypothesized that SG would lead to a decrease in vBMD and differential changes in MAT.12-month prospective study in 52 adolescents with moderate-to-severe obesity (38F, mean age:17.5±2.2 years, mean BMI:45.2±7.0 kg/m 2), 26 subjects before and after SG and 26 nonsurgical controls.Lumbar vBMD by QCT; MAT of the lumbar spine, femur and tibia by 1H-MRS; abdominal fat and thigh muscle by MRI.Adolescents lost 34.1±13.1 kg after SG vs 0.3±8.4 kg in the control group (p&0.001). Lumbar vBMD decreased in the SG group (p=0.04) and this change was associated with a reduction in weight and muscle area (p&0.05) and an increase in lumbar MAT (p=0.0002). MAT of the femur and tibia decreased after SG vs. controls (p&0.05), however, the differences were no longer significant after controlling for change in weight.SG in adolescents decreased lumbar vBMD associated with an increase in lumbar MAT and decrease in extremity MAT. This demonstrates differential changes of regulated MAT in the lumbar spine and constitutive MAT in the distal skeleton in adolescents in response to SG.