Photon absorptiometry was used to measure skeletal mass in the proximal femur, lumbar spine, and distal radius in 19 females with hypoparathyroidism after operation for either thyroid carcinoma or hyperparathyroidism. Healthy subjects as well as normocalcemic patients who had undergone the same surgical procedure without developing hypoparathyroidism were used as controls. Skeletal mass was measured after a mean postoperative time of 13 and 10 yr in patients operated on for thyroid carcinoma and hyperparathyroidism, respectively. Bone mass was 10-32% greater in hypoparathyroid patients than in controls. In patients with retained parathyroid function after total thyroidectomy and surgical treatment of hyperparathyroidism, bone mass did not differ from that in age-matched healthy controls. Long term T4 medication in doses that suppressed endogenous TSH production was not associated with a decreased bone mass. Reduced PTH production, vitamin D treatment, and calcium supplementation may all have contributed to the increased bone mass found in the patients with postsurgical hypoparathyroidism.