OBJECTIVE Bone mass is often reduced in patients with primary hyperparathyroidism (pHPT) and is usually partially reversible after parathyroidectomy. However, site specific and overall skeletal benefits of surgery in mild asymptomatic pHPT remain uncertain. METHODS Cross-sectional and longitudinal studies. METHODS Fourteen patients (12 women and 2 men) with pHPT. METHODS Baseline bone mass was assessed at the lumbar spine, left hip and whole body using dual-energy X-ray absorptiometry, and at the left os calcis using broad-band ultrasound attenuation. Changes in bone mass, serum intact PTH and osteocalcin, and urinary pyridinium cross-link excretion were recorded in 10 patients followed for 6 months after surgery. RESULTS (1) Cross-sectional study: Baseline measurements at the lumbar spine and hip were inversely related to both the serum PTH concentration and the weight of the parathyroid gland removed at surgery. (2) Longitudinal study: Six months after adenectomy, bone mass had increased significantly at the femoral neck, greater trochanter, whole body and os calcis, but not at the lumbar spine or Ward's area. Serum PTH, osteocalcin and pyridinium cross-link excretion all fell significantly after surgery. The percentage increment in whole body bone mineral content at 6 months was proportional to the baseline PTH. CONCLUSIONS In primary hyperparathyroidism, preoperative reductions and post-operative gains in bone mass are proportional to the initial serum PTH concentration. Mild primary hyperparathyroidism probably does not cause appreciable bone loss at clinically relevant fracture sites such as the spine and hip, and in such cases the overall skeletal benefits of surgery are likely to be negligible.