BACKGROUND: Sanfilippo syndrome, or mucopolysaccharidosis (MPS) type III, is a rare lysosomal storage disease, resulting from errors in the catabolism of heparan sulphate. Aim: To evaluate bone turnover and bone mineral density (BMD) in MPS type III patients. DESIGN: Clinical and observational study. METHODS: We evaluated serum markers of bone formation or resorption, and measured BMD using dual-energy X-ray absorptiometry (DEXA), in three patients with MPS type III. RESULTS: Serum vitamin D were low, and BMDs greatly reduced at lumbar and femoral sites, indicating the possibility of osteoporosis and osteomalacia. DISCUSSION: These skeletal effects probably result from nutritional deficiencies and inability to walk, rather than from the genetic defect itself. Secondary skeletal involvement in patients with MPS type III may represent a considerable cause of morbidity, and requires interventions to reduce the risk of pathological fractures.