Dual-photon bone densitometry of the lumbar spine and hips was performed in 11 patients (9 females) with asymptomatic primary hyperparathyroidism and in 11 control subjects. The age was 54.7 +/- 14.8 years. The asymptomatic state of the disease was established by the absence of clinical or radiologic evidence of renal, digestive or bone involvement. 10 of 11 patients had histologic confirmation of the disorder, in addition to the biochemical and RIA findings which were present in all. Serum calcium ranged from 10.6 to 11.8 mg/dl, and P levels were decreased in 7 subjects. Alkaline phosphatase levels were elevated in 7 patients. Urinary calcium output was 341 +/- 216 mg/24 h. Bone density was 16% lower at the lumbar spine and 20% lower at the hip in patients compared to controls. In 5 patients bone density was lower than the level associated to increased risk for fractures (compared to 2 controls). No correlation existed between bone density and biochemical indices of hyperparathyroidism. Thus, decreased bone density and increased risk of fracture may exist in patients with hyperparathyroidism even at the asymptomatic stage.