Objective: To evaluate fracture rate and bone mineral density (BMD) and body composition in children with acute lymphoblastic leukemia (ALL) treated with dexamethasone-based chemotherapy. Study design: Children with ALL (n = 61) participated. At diagnosis, during therapy, and one year after cessation of therapy, BMD and body composition were measured using dual energy X-ray absorptiometry of lumbar spine (LS) and total body (TB). Serum markers of bone turnover were assessed. Results: BMD_LS was significantly reduced at diagnosis, and remained low during therapy. BMD_TB was normal at diagnosis, with a fast decrease in the first 32 weeks, in which chemotherapy was relatively intensive. Apparent ("volumetric") BMD_LS was also reduced, but this did not reach significance at diagnosis and follow-up. Bone formation markers were reduced at diagnosis; formation as well as resorption markers increased during treatment. Fracture rate was 6 times higher in ALL patients compared with healthy controls. Lean body mass was decreased at baseline. Percentage of body fat increased significantly during therapy. After ALL treatment was completed, BMD and body composition tended to improve. Conclusions: Children with ALL are at risk for osteopenia because of the disease itself and the intensive chemotherapy. Fracture rate increases substantially, not only during but also shortly after treatment. (J Pediatr 2002;141:204-10)