987 patients with Paget's disease were treated with disodium etidronate at an oral dose of 5 mg/kg/d and followed up for an average of 5.2 years: each course of treatment lasted 6 months and was repeated once or several times. 76 per cent of the patients with symptoms had improved by the end of the follow up period, and 85 per cent of the asymptomatic patients showed none of the complications that might have been expected preventive treatment with etidronate not been undertaken. The course of the disease deteriorated in 28% of the patients between start and finish of the study, but the result was sometimes incomplete or in some doubt. For patients receiving treatment, the annual incidence of coxarthropathy was 1 per cent. In patients with medullary involvement, indicated by spasmodic paraparesis during the initial assessment, oral disodium etidronate always had a curative effect. In patients with cranial involvement, the condition worsened fairly frequently, with hypoacusis arising or deteriorating in 29 per cent of the patients. Twenty three fractures occurred, including 8 in pagetic bone, with an incidence of respectively 5 and 1.7 per cent patient-years: the risk of fracture did not seem to be increased when etidronate was used at a low dose. Among the side effects observed, there was a delayed increase in bone pain in 12.5 per cent of the patients, the symptoms generally resolving except in one case where a fracture occurred.(ABSTRACT TRUNCATED AT 250 WORDS)