Idiopathic hyperphosphatasia is a rare autosomal recessive bone disorder, characterized by excessive bone resorption and bone formation. The radiographic appearances include widening of the diaphyses, vertebral osteoporosis, acetabular protrusion, and thickening of the skull vault. There is considerable variability in phenotype, with some cases diagnosed in infancy and others in later childhood. Most cases appear to arise from inactivating mutations in the gene encoding osteoprotegerin, a product of osteoblasts that is critically involved in osteoclastogenesis. Treatment with inhibitors of bone resorption (calcitonin or bisphosphonates) is successful in ameliorating some aspects of the disorder.