Quantitative bone scintigraphy (QBS), which measures 99mTc-MDP uptake expressed as percent of injected dose per cc, indicates bone metabolism. It is measured in the bones of patients before and after radiation treatment and then compared to normal controls. QBS was performed in a group of 22 normal individuals and was measured twice, 2-10 mo (mean 4.9) apart. There was no significant difference between the two measurements. QBS was performed also in 28 patients before, immediately after and at certain time intervals after radiation therapy for cancer. Both the irradiated and the nonirradiated bones showed significant decreases in bone metabolism at 2-18 mo (mean 8.8) after irradiation. In addition, increases and decreases of 99mTc-MDP uptake were similar in the irradiated and in the nonirradiated bones, and there were significant correlations of the QBS values in the different bones of each individual patient. The etiology of the changes in bone metabolism in the nonirradiated bones is not yet fully understood, but it appears to be the result of a systemic effect of radiation.