Eleven patients with Paget's disease treated with sodium etidronate 20 mg/kg/day for two and four weeks showed significant reductions in plasma alkaline phosphatase activity and urinary hydroxyproline excretion, both of which are biochemical markers of bone turnover. After four weeks of treatment, however, histological examination of iliac crest biopsy samples showed that despite a rapid reduction in bone resorption there was an appreciable mineralisation defect; even after only two weeks' treatment the abnormalities in bone formation persisted for up to 10 weeks. The adverse effects of sodium etidronate on mineralisation cannot be dissociated from its beneficial effect on resorption even when it is given for short periods.