We studied nine patients with Paget's disease affecting the skull or facial bones, who were subsequently treated with either dichloromethylene diphosphonate (clodronate) or ethylene-1-hydroxy-1,1-diphosphonate (etidronate). Long-term treatment induced a clinical and biochemical improvement in eight, and this was associated with a reduction in maxillary or skull volume as assessed by quantitative stereophotogrammetry. The one patient whose disease was resistant to treatment with diphosphonate, showed no change in maxillary shape. These studies suggest that the long-term control of disease activity attained with diphosphonates, results in the improvement of skeletal deformity.