Biochemical bone markers and bone mineral density (BMD) in spine, hip, and forearm were measured, before surgery, in 30 patients with mild to moderate primary hyperparathyroidism (PHP) (25 women and 5 men; mean age 54 +/- 12 years, range 26-73 years) and compared with normal controls. A group of 291 healthy adults (181 women and 110 men) served as controls for BMD. A smaller group of 30 normal individuals (25 women and 5 men; mean age 54 +/- 12 years; range 26-74 years) were used as matched normal controls. Parameters of bone formation (s-osteocalcin, s-alkaline phosphatase activity, and s-bone isoenzyme alkaline phosphatase activity) and bone resorption (s-type-1 collagen telopeptide) were considerably increased in patients with PHP compared with normal controls (p < 0.01 for all parameters). BMD was found to be reduced in the hip (trochanteric: 95.1 +/- 14.7% of expected, p < 0.05; intertrochanteric: 95.2 +/- 13.8% of expected, p < 0.05), and the forearm (proximal: 93.3 +/- 12.2% of expected, p < 0.05; mid: 91.8 +/- 11.6% of expected, p < 0.001; distal: 90.7 +/- 13.1% of expected, p < 0.001). Spine BMD was found significantly reduced in premenopausal (87.8 +/- 7.6% of expected, p < 0.05) but not in postmenopausal patients, and although normal women showed a decrease in spinal BMD with increasing age this was not found in the PHP women. Forearm BMD was reduced in both pre- and postmenopausal patients (distal forearm: 86.7 +/- 12.2% of expected, p < 0.05; 87.6 +/- 12.1% of expected, p < 0.01, respectively). It was concluded that Danish patients with mild or moderate PHP have only small reductions in BMD. The bone loss is mainly found in the appendicular skeleton.