Bone loss and the serum markers of bone metabolism were studied in 22 patients with primary hyperparathyroidism and 108 patients with renal hyperparathyroidism. The parameters of bone loss were bone mineral density in the distal radius and lumbar vertebrae, measured by dual energy X-ray absorptiometry, and bone mass index (sigma GS/D) and the metacarpal index, in the second metacarpal bone, measured by the digital image processing method. Alkaline phosphatase (AIP), intact osteocalcin (OC), and the carboxyterminal propeptide of type I procollagen (PICP) were measured as serum markers of bone formation, while tartrate-resistant acid phosphatase (TRACP) and the carboxyterminal pyridinoline cross-linked telopeptide of type I collagen (ICTP) were measured as serum markers of bone resorption. Bone loss and elevated markers of bone metabolism were observed both in patients with skeletal symptoms and in those without. Furthermore, the decrease in the cortical bone mass was more predominant than that of the trabecular bone. As markers of bone formation, AIP and OC seemed to be more sensitive than PICP, and as markers of bone resorption, ICTP appeared to be more sensitive than TRACP. Thus, a close correlation was observed between bone loss and the markers of bone formation and resorption.