Smoking is a risk factor for osteoporosis. Nicotine and nonnicotine tobacco smoke components have been shown to depress osteoblast activity in a number of in vitro and animal studies. To determine whether smoking is associated with depressed osteoblast activity in humans, we measured serum osteocalcin levels (using a radioimmunological method based on an antibody to human osteocalcin) in 24 male or female smokers and 24 matched nonsmokers. Overall, osteocalcin levels were significantly lower in smokers (15 +/- 6.95 ng/ml) than in nonsmokers (21.27 +/- 8.34 ng/ml) (p = 0.007). The difference between smokers and nonsmokers was significant in males (15.3 +/- 4.5 vs 23.27 +/- 9.7; p = 0.02) but not in females (16.27 +/- 8.9 vs 19.45 +/- 6.7; p = 0.2). These data suggest that smoking may induce osteoblast depression, either directly or via hormonal changes.