To evaluate the effects of primary hyperparathyroidism (PHPT) on bone mass and structure, we have studied the iliac crest biopsies of 27 patients, 10 males (28–68 yr old) and 17 females (26–72 yr old) with mild PHPT after in vivo tetracycline labeling. All patients had mild hypercalcemia in the absence of any other cause and elevated levels of PTH without radiological evidence of bone disease. Static parameters of bone turnover (osteoid surface, osteoid volume, and eroded surface) were elevated in both men and women compared to normal values; the midmolecule RIA for PTH (PTH_MM) was positivelycorrelated with osteoid surface (r = 0.44; P < 0.025) and eroded surface (r = 0.58; P < 0.005). Dynamic parameters of bone turnover (mineralizing surface, expressed as double plus half single labeled surface, and bone formation rate at tissue level) were elevated compared to normal values; PTH_MM was positivelycorrelated with double plus half single labeled surfaces (r = 0.33; P < 0.05) and with bone formation rate at the tissue level (r = 0.37; P < 0.05). The mineral apposition rate was within the limits of normal values and positively correlated with PTH_MM (r = 0.34; P < 0. 05). Histomorphometric parameters of bone structure [cancellous bone volume (BV/TV), trabecular thickness (Tb. Th), trabecular number (Tb.N), trabecular separation (Tb.Sp), cortical thickness (Ct.Th), and total bone density (TBD)] were compared to those in 20 autopsy control subjects, 12 men (33–60 yr old) and 8 women (27–75 yr old). BV/TV and Tb.N were significantly higher in PHPT patients than controls (P < 0.02 and P < 0.001, respectively). Tb.Sp was significantly lower in PHPT patients than controls (P < 0.001), whereas Tb.Th was not significantly different between PHPT patients and controls. Ct.Th was significantly lower in PHPT patients than in controls (P < 0.001), whereas TBD was not significantly different between the two groups. BV/TV was negatively correlated with age in both controls and PHPT patients. Tb.N showed a negative correlation and Tb.Sp a positive correlation with age i n controls (r = −0.47; P < 0.05 and r = 0.52; P < 0.02, respectively), but they were not significantly dependent on age in PHPT patients. Tb.Th, while showing no significant age-related change in controls, was negatively correlated with age in PHPT patients (r = −0.42; P < 0.05). Ct.Th correlated negatively with age in controls (r = −0.46; P < 0.05), but showed no age dependency in PHPT patients. In conclusion, our data showed that compared to normal subjects, patients with PHPT have increased bone turnover, thinning of cortical bone, and increased cancellous bone volume. We propose that a different mechanism of age-related bone loss in PHPT allows the striking maintenance of well connected trabecular plates, and that increased levels of PTH, therefore, may have a protective effect on cancellous bone.