Paget's bone disease is rare in young adults. Severe osteolytic Paget's bone disease in a 28-year-old man was found to respond, clinically, biochemically, and radiographically, within one month to daily subcutaneous injections of 0.5 mg of synthetic human calcitonin. After two years of therapy, he remains asymptomatic and has no biochemical evidence of Paget's bone disease while receiving injections three times a week. Despite aggressive disease, young patients may rapidly demonstrate the same beneficial response to synthetic human calcitonin therapy as has been observed in middle-aged or elderly patients with Paget's bone disease.