Osteocalcin is a 49 amino acid non collagenous bone matrix protein which is synthesized by the osteoblasts. The serum levels of osteocalcin have been found to be a specific biochemical parameter of bone formation. We determined the serum levels of osteocalcin, parathyroid hormone, calcitonin and alkaline phosphatase as well as the 2 hour fasting hydroxyproline excretion in 26 patients with postmenopausal osteoporosis and in 24 postmenopausal control subjects. Serum levels of osteocalcin were significantly lower in the patients with postmenopausal osteoporosis than in the control subjects (p less than 0.002). In contrast, serum levels of parathyroid hormone, calcitonin, alkaline phosphatase and the 2 hour hydroxyproline excretion in the patients with postmenopausal osteoporosis and the control subjects were not statistically different. Our data give evidence of a decreased bone formation in patients with postmenopausal osteoporosis.