Chronic idiopathic hyperphosphatasia (CIH), also known as juvenile Paget's disease, is characterized by increased bone turnover, persistently elevated serum alkaline phosphatase concentrations and progressive bone deformities, The pathogenesis of the disease is unknown. Currently, there is no specific treatment and agents that reduce bone turnover have been tried in some cases with limited success. In this report, we present our experience with alendronate treatment in a 17 year-old boy with CIH, Ten weeks of treatment with alendronate resulted in marked clinical improvement and normalization of serum alkaline phosphatase activity. Serum osteocalcin and urinary deoxypyridinoline levels were decreased approximately 50% compared to pretreatment values, indicating decreased bone turnover rate. Alendronate seems to be a promising and safe agent for treatment of CIH.