The objective of this study was to evaluate the long term comparative outcome of intermittent courses of therapy with low-dosage tiludronic acid (400 mg/day) versus etidronic acid in patients with Paget's disease of bone. The long term outcome was evaluated by means of a model, as the number of complications avoided during a 5-year period. Only major complications were considered (fractures of long bones, spinal cord dysfunction, osteoarthritis of the hip), which can be cured by adequate medical intervention, and may thus be valued in terms of direct costs. Deafness was not considered and was not valued. The various additional direct costs incurred (treatment and follow-up) and avoided (complications avoided) were assessed using an extrapolation model, based on the results of a clinical study. A standard costing methodology was used, according to the patterns of care currently in use in France. Costs were calculated from a societal perspective, including social security and private insurance fund reimbursements, and out-of-pocket expenses. These were estimated in French francs (F: 1994 values). The estimated mean treatment cost per patient for the treatment of Paget's disease and its complications over a 5-year period was F34,198 with etidronic acid, and F30,754 to F36,422 with tiludronic acid, depending on whether an optimistic or a conservative view of the efficacy of tiludronic acid was assumed. Despite its higher acquisition cost, the use of tiludronic acid was less expensive in the treatment of Paget's disease and its complications, compared with the use of etidronic acid, when an optimistic view of the efficacy of tiludronic acid was assumed.