We studied the effect of the intravenous administration of the bisphosphonate APD in 9 patients with Paget's bone disease. The medication was given in a daily dose of 25 mg for 7 days in 0.9% saline infusion over 2 hours. At the end of treatment a significant fall of serum calcium and phosphate was observed. The urinary excretion of calcium decreased markedly and the serum levels of the mid-molecule PTH fragment increased from (mean +/- SE) 85 +/- 11 to 122 +/- 16 pg/ml (p less than 0.05). A marked and rapid decline in the hydroxyprolinuria was observed from 297 +/- 61 mg/24 h to 194 +/- 51 mg/24 h (p less than 0.01); meanwhile the serum alkaline phosphatase decreased from 102 +/- 22 to 84 +/- 21 KAU (p less than 0.05). The effect of ADP on suppression of hydroxyprolinuria varied markedly from +1 to -81% and was negatively related to the basal hydroxyprolinuria (r = -0.90; p less than 0.001). The duration of the bone turnover suppression was short. A relapse greater than 30% in hydroxyprolinuria was observed in 6 of 8 patients 2 to 3 months after APD withdrawal. The short-term intravenous administration of ADP is a useful means to rapidly suppress the activity of Paget's bone disease. However, further studies should determine the optimum dose, the length of treatment, and the need to associate oral therapy to induce a prolonged remission.