Several agents are available for treatment of Paget's disease of bone, but their long-term use may be limited by resistance or adverse effects. I have treated two patients with exceptionally severe polyostotic Paget's disease (serum alkaline phosphatase values 25- to 30-fold above normal), in whom salmon calcitonin, sodium etidronate, and plicamycin each had become ineffective or could not be used. Pamidronate (3-amino-1-hydroxypropylidene-1,1-bisphosphonate) decreased serum alkaline phosphatase to normal or near-normal values and almost completely relieved symptoms, without recognizable adverse effects, except for transient mild hypocalcemia. The symptomatic and biochemical responses were maintained through 2 years of treatment and for more than 6 months after the treatment was discontinued. Thus, pamidronate can be very effective in severe, resistant cases of Paget's disease. Further study of this potent agent is needed to define the optimum regimen for maximum effectiveness and minimal long-term toxicity.