Of 58 patients with Paget's disease treated with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) in doses of 20 and 10 mg/kg/day, a group of 20 patients with the oligostotic form of the disease showed a significantly greater incidence of complications, such as worsening of bone pains, when compared with a group of 38 patients with the polyostotic disease (P < 0.05). The group of 32 patients that received 20 mg/kg/day showed a greater although not significant incidence of clinical complications than the group of 26 patients treated with 10 mg/kg/day. Bone biopsies performed in one polyostotic and three oligostotic cases who suffered episodes of bone pain worsening during treatment with 20 mg/kg/day disclosed a severe osteomalacia. Both groups treated with 20 and 10 mg/kg showed a highly significant decrease of urinary hydroxyproline (THP) excretion and of serum alkaline phosphatase (P < 0.01) after two and six months of treatment although the trend comparison between doses was not significant. It is suggested that the dose of EHDP should be related to the extent of the disease.