Serial radionuclide bone scans (n = 96) of 40 patients with Paget's disease were studied and tracer uptake graded using a four-point subjective scale. In nine patients studied without treatment the serial scan appearance improved in some, remained unchanged in others and deteriorated in yet others. Similarly individual lesions demonstrated increased, decreased or unchanged tracer uptake. One lesion disappeared without treatment but no new lesions developed. After treatment with aminohydroxypropylidene bisphosphonate (APD) or ethanehydroxy bisphosphonate (EHDP) the overall scan appearance improved in the majority of patients treated (n = 34), but individual lesions demonstrated increased (n = 20), unchanged (n = 79) or decreased uptake (n = 86). The mean reduction in tracer was 32% and 20 lesions reverted to normal intensity. The site of the lesion or the initial intensity of tracer uptake did not influence response to therapy. New lesions developed in five patients after therapy. There was variable correspondence between scintigraphic changes and biochemical parameters of response. From our study we conclude that serial bone scans must be used with caution when interpreting the response of Paget's disease to therapy.