Bone metabolism was assessed in vivo and noninvasively using quantitative SPECT. The effect of endocrine abnormalities on bone metabolism was studied in 27 patients with primary hyperparathyroidism (HPT) and 12 patients with thyrotoxicosis (TTX). Quantitative bone scintigraphy (QBS) values of 99mTc-MDP uptake were compared to normal values matched for sex and age. Bones with significantly increased QBS values indicating increased bone metabolism were identified in the two patient groups. Fifty-one percent of the bones in patients with HPT and 78% in patients with TTX showed significantly increased QBS values. Increase in bone metabolism was highest in the femoral shaft. Seven patients with HPT and five with TTX were successfully treated. Six patients with HPT and four patients with TTX showed significant decrease of bone metabolism with normal QBS values after three months. The results indicate that QBS can be used to evaluate bone metabolism and its response to treatment in individual bones in patients with endocrine abnormalities.