To evaluate the effect of bone formation on alveolar bone destruction in periodontitis, we selected 20 patients with severe chronic periodontitis (CP), and 9 patients with rapidly progressive periodontitis (RPP). Their serum osteocalcin was determined by radioimmunoassay (RIA). The serum osteocalcin in 65.0% patients with CP and 66.7% patients with RPP was lower than that in normal persons. It is suggested that osteoblastic activity and bone formation ability of periodontitis patients are lower than those of normal persons. The alveolar bone destruction in periodontitis results from not only increased osteoclastic activity and bone resorption but also decreased osteoblastic activity and bone formation.