Bone scintigraphy with 99mTc-MDP was performed on 8 patients with Paget's disease of bone. The radionuclide uptake by all the involved lesions was markedly increased, even in subclinical lesions without pain. Bone scintigraphy with 99mTc-phosphorous compounds were thought to be the most simple and sensitive technique to define the precise extent of the lesions, and to detect asymptomatic occult cases with Paget's disease. Possible A-V shunt was estimated in 3 cases by measuring the radioactivity over the lungs after the injection of 99mTc-MAA through a catheter into an artery which supplied the lesion. A-V shunt was calculated as 14.5%, 10.0% and 12.0%, respectively. An uptake study of 99mTc-MDP was attempted to quantify the effect of calcitonin treatment using a gamma camera combined with a computer. An "uptake ratio" was obtained for each lesion by dividing the count rate over the bone lesion by that over the control bone. Three cases of Paget's disease were treated with synthetic eel calcitonin analogue ([Asu1,7] E-CT) in a dose of 40 MRC unit per day. The effectiveness of CT therapy was evaluated by the X-ray film, the serum alkaline phosphatase activity (S-Al-P), the serum phosphate level, the serum calcium level and the "uptake ratio". No remarkable changes were obtained on bone X-ray films at one year after the initiation of the CT treatment in all cases. The S-Al-P levels did not show significant difference in the 2 cases, in which the S-Al-P levels were within the normal range before the treatment. In all cases, however, the "uptake ratio" of the diseased bone fell remarkably within the first three months and the rate of the fall was parallel to the decrease in the bone pain. It was considered that the "uptake ratio" on bone scintigraphy offered the most sensitive and reliable information in evaluating the CT treatment for Paget's disease.