The effects of EHDP (20 mg/kd/day) and APD (4.5 mg/kg/day) given for three months to patients with severe symptomatic Paget's disease have been compared in an open trial of 17 patients. Both drugs were equally effective in producing a prompt reduction in pair scores, urine hydroxyproline, and serum alkaline phosphatase levels. The remission was maintained for a variable period after stopping treatment. Both drugs were well tolerated, and a one-month course of either drug was not effective. Comparison with published responses from previous studies indicates that EHDP given at this dose as a relatively short course is more effective than a lower dose for a longer period of time; the present study does not suggest that APD has significant advantages.