Most patients with primary hyperparathyroidism have reduced radial and preserved vertebral bone density. We have identified a subset of patients with low lumbar spine bone density at diagnosis. This study assessed the effect of parathyroidectomy (undertaken based upon accepted surgical guidelines) or nonintervention on bone mineral density (BMD) in these patients. Twenty-two of 143 (15%) patients with mild primary hyperparathyroidism had lumbar spine BMD more than 1.5 SD below the mean for an age- and sex-matched population (z-score). Fourteen underwent parathyroidectomy, whereas 8 were followed with no intervention. All had annual BMD measurements for 4 yr after enrollment or after surgery. After parathyroidectomy, there was a brisk sustained rise in lumbar spine BMD [yr 1, 15 +/- 3% (P < 0.005); yr 4, 21 +/- 4% (P < 0.01)]. In those followed without surgery, BMD did not change significantly at any site. Postmenopausal women showed the same pattern as the cohort as a whole, i.e. increased BMD after surgery [yr 1, 13 +/- 3% (P < 0.01); yr 4, 16 +/- 5% (P < 0.01)], but no worsening was found with nonintervention despite the passage of years in the menopause. We conclude that parathyroidectomy markedly improves lumbar spine BMD in patients with vertebral osteopenia. It is proposed that reduced cancellous bone density should become a new indication for surgery in primary hyperparathyroidism.