The effects of treatment with aminobisphosphonate (APD) have been studied in a large and well-defined group of patients with Paget's disease over a period of seven years. Particular attention is given to the pharmacology of the drug, to methods of assessment of efficacy, and to the quality and the long-term persistence of the treatment results. These studies are compared to previously reported studies on bisphosphonates (P-C-Ps). The data suggest that the efficacy of P-C-Ps in Paget's disease results from a physiologic adaptation of all cellular processes involved in bone metabolism to a primary inhibition of bone resorption. The prolonged persistence of remissions may indicate that this is associated with disappearance rather than suppression of pathogenic material. If the low specific toxicity of the new generations of P-C-Ps is confirmed, it will be possible to induce complete and prolonged remissions through short oral or parenteral treatment courses that are associated with minimal side effects. Early institution of treatment in selected patients may prevent the development of deformity, fracture, and pain.