The aim of the current study was to analyze the osteocalcin level and radiographic density during distraction osteogenesis in order to investigate the role of osteocalcin in monitoring bone formation during callus distraction. Lengthening of the right tibia by 25% was performed in 12 beagle dogs by callus distraction after osteotomy and application of a ring fixator. Distraction was started on the 5th postoperative day, with a distraction rate of 0.5 mm twice a day, and was ended after 25 days. Blood samples and x-rays of the callus distraction segment were obtained preoperatively and once a week until day 55 after operation. A digital radiograph analysis system was used to determine the bone density of the callus distraction segments. The serum parameters of osteocalcin were evaluated by radioimmunoassay. The radiographic bone densities during the distraction phase increased during the distraction period and markedly increased during the consolidation period. A similar trend was observed for osteocalcin, whereby the coefficient of correlation between these two parameters was, on average, 0.68 +/- 0.11. However, the radiographic bone density measurements, as well as the osteocalcin levels, showed large variation between different animals. Therefore, our results suggest that valuable information about bone formation during distraction osteogenesis can be obtained via serum osteocalcin levels, even though it seems that time sequence monitoring is more favorable than the determination of absolute values.