A new dynamic classification of Paget's disease is proposed, incorporating both the radiographic phases of bone remodelling and the scintigraphic findings. Osteolytic, mixed and osteoblastic phases are associated with increased scintigraphic activity, while the osteosclerotic phase of remodelling is associated with normal or diminished activity and an osteoblastic radiographic appearance. Abnormal modelling of bone leading to deformity is produced by accelerated apposition or absorption at the periosteal and endosteal envelopes of the bone. In 112 patients with symptoms from Paget's disease, 527 lesions were classified. The most frequent remodelling phase was the mixed one and the most common modelling state was bone expansion with endosteal and periosteal apposition. Of 88 patients treated medically, 12 had lesions which progressed to increased bone formation without a change in modelling, and the active lesions in seven patients became inactive. Prolonged treatment with disodium etidronate led to progressive osteopenia in 11 patients.