Fourteen patients with Paget's disease of bone were treated with disodium etidronate in doses of 5 to 7 mg/kg per day. Skeletal blood flow (SBF), was measured by the modified 18F clearance technique of Wootton et al. (1976) before treatment and again during treatment. In 10 patients restudied 3-4 months after the start of therapy, SBF had fallen by a mean of 21% of the initial value, and the individual differences correlated well with the individual reductions in serum alkaline phosphatase (r = 0.77, P less than 0.01). The results were similar to those seen in an earlier study in patients treated with calcitonin. However, no early reduction in SBF was seen in six repeat studies performed at the end of the second week of treatment, in contrast with our previous findings with calcitonin.