Neurofibromatosis type 1 (NF1) is a common genetic disorder affecting 1 in 3000 live births. It is well documented to be associated with bony deformities and other orthopaedic problems. Based on our observation that NF1 patients undergoing orthopaedic surgery often had osteopenic bone, we performed a study to assess the bone mineral density of a cohort of children with NF1 without orthopaedic defects.Twenty-three patients were recruited from the neurofibromatosis clinic. The bone mineral density of the total body, lumbar spine, and proximal femur was measured using dual-energy x-ray absorptiometry. Quantitative ultrasound was used to measure broadband ultrasonic attenuation at both heels. The group's mean dual-energy x-ray absorptiometry sex- and age-matched Z scores were below normal (-0.8 +/- 1.1, -0.8 +/- 1.2, -0.7 +/- 0.8, -0.6 +/- 1.1, -0.6 +/- 0.9, -0.6 +/- 1.1 for the total body, arms, legs, lumbar spine, and right and left femoral neck, respectively; all P < 0.01). Although some individuals had normal bone mass, 30% had total body Z scores below -1.5. The mean heel broadband ultrasonic attenuation Z score was also lower than normal (-0.8 +/- 0.6; P < 0.001). Children with NF1 have a general tendency toward osteopenia, suggesting an abnormal underlying bone phenotype. This may be relevant when considering operative intervention and, if better understood, may partially explain poor bone healing associated with NF1.