The consequences of bone disease in primary hyperparathyroidism (PHPT) are still a matter of discussion. We studied transiliac bone biopsies of 24 patients (9 men and 15 women) with mild to moderate PHPT, proven by surgery. Histomorphometric data were compared with control values obtained from autopsy bone samples. The biopsies of the PHPT patients were characterized by increased trabecular bone volume (P < 0.01) and surface (P < 0.02), increased osteoid volume and increased osteoid and resorption surfaces (P < 0.001). Cortical porosity showed a non-significant increase in the PHPT patients. In the patients the fasting serum concentrations of parathyroid hormone (PTH(1-84)) correlated positively with cortical porosity (r = 0.50, P = 0.02), osteoid volume (r = 0.40, P = 0.05), osteoid surface (r = 0.50, P = 0.01), and eroded surface (r = 0.59, P < 0.01). It is concluded that moderate PHPT is associated with preservation of trabecular bone, whereas cortical bone may be lost in these patients. Cortical porosity and the parameters of bone turnover correlate well with the fasting serum PTH(1-84) concentration.