Iliac crest bone biopsies from 39 patients (23 women, 16 men) with untreated primary hyperparathyroidism (PHP) were examined. Static histomorphometric parameters for cortical bone was compared with values obtained from 39 age- and sex-matched normal controls. Thirty-five of the patients were double-labeled with tetracycline before biopsy. The cortical remodeling cycle was reconstructed in these patients and compared with 24 sex-matched but younger normal controls. In 9 patients, a second intact bone biopsy was obtained 6-12 months after successful surgery for PHP. In these patients, pre- and post-treatment biopsies were compared. Cortical porosity was increased in PHP (p < 0.001) due to a high turnover state with expansion of the remodeling space (p < 0.001). The net balance of the remodeling cycle on the haversian surface was found unchanged in PHP, since the average haversian canal diameter remained unchanged. In the female PHP-patients, a significant increase in the final resorption depth (p < 0.01) was balanced by a proportional increase in the wall width (p < 0.001). Among all PHP patients, no significant decrease in cortical width was found. However, a significant reduction in absolute (p < 0.05) and fractional cortical width (p < 0.05) was seen in postmenopausal PHP women giving rise to a cortical bone loss. Reconstruction of the cortical remodeling cycle revealed a doubling in activation frequency from 0.6#/year in normals to 1.2#/year in PHP (p < 0.001), giving rise to a high turnover state. Surgical cure of PHP was accompanied by marked changes in cortical bone metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)