This investigation was undertaken to determine whether the preservation of bone mass in patients with mild primary hyperparathyroidism (PHPT) could be detected when measuring spine density in the lateral projection. We compared the bone mineral density (BMD) of L2-L4 utilizing the posterior-anterior (PA) and lateral projections in postmenopausal patients with PHPT and in a group of 27 postmenopausal normal women. Thirty-three consecutive postmenopausal patients with PHPT were studied; 25 were asymptomatic whereas the remaining 8 suffered complications related to the disease. Based upon the criteria established by the Consensus Conference on the Management of Asymptomatic PHPT, only 10 of the 25 asymptomatic patients could be considered affected by mild disease; the remaining patients were classified as having moderate disease. Patients with mild disease had mean lateral total BMD values (0.682 +/- 0.113 g/cm(2)) significantly higher than normal women (0.588 +/- 0.076, p<0.02) and patients with moderate disease (0.599 +/- 0.077, p<0.05). There were significant differences among the three groups in both PA L2-L4 and L1-L4 levels: patients with mild disease had significantly higher mean BMD values than patients with moderate disease and normal women, when either three or four vertebrae were considered. Interestingly, at this latter site, patients with moderate disease had significantly ( p<0.05) lower values than normal women. Our results indicate that patients with mild PHPT have a preservation of vertebral mass when compared with the other hyperparathyroid patients and normal women, when taking into account both the mainly trabecular portion and the whole vertebra. The finding that when the PA projection was assessed, BMD values of patients with moderate disease were significantly lower than those of normal women, might be attributed to the detrimental effect of raised parathyroid hormone levels on the cortical component of the vertebral body.