Tobacco was found to be a risk factor for osteoporosis, mainly in postmenopausal women. We studied the effect of smoking on bone mineral density (BMD) and bone turnover in a cohort of 719 men, aged 51-85 yr, composed of 83 current smokers, 405 former smokers, and 231 men who never smoked. Most current and former smokers were moderate smokers (median, 10 cigarettes/d). Current smokers were younger, thinner, and drank more coffee and more alcoholic beverages. After adjustment for age, body weight, alcohol intake, and caffeine intake, current and former smokers had similar BMD, except at the forearm. Former smokers had lower BMD compared with never-smokers at most skeletal sites. Men who had smoked more than 7120 packs (third quartile) had lower BMD of total hip (P < 0.01) and distal forearm (P = 0.03) compared with men in the 2 lower tertiles. In the 3 groups, levels of bone formation markers did not differ. After adjustment for confounding variables, levels of urinary markers of bone resorption (beta-isomerized C-terminal telopeptide, free and total deoxypyridinoline) were higher in the current smokers than in former smokers and never-smokers. Concentrations of T, total 17beta-E2, and androstenedione were higher, whereas that of 25-hydroxyvitamin D was lower, in current smokers. When men were divided according to tertiles of body weight, increased bone resorption, decreased BMD and biochemical indexes of secondary hyperparathyroidism were observed in current smokers in the lowest tertile of body weight (<75 kg) compared with the never-smokers, but not in men in the two highest tertiles of body weight. Current smokers had a higher prevalence of vertebral deformities after adjustment for age and body weight (13% vs. 5%; P < 0.005). In summary, in moderate smokers with low body weight (<75 kg), increased bone resorption, not matched by increased bone formation, results in decreased BMD and an increased prevalence of vertebral deformities. In this group, low serum 25-hydroxyvitamin D and secondary hyperparathyroidism may explain, at least partly, the effect of tobacco on bone turnover. In former smokers, bone resorption is not increased, but BMD remains lower compared with that in never-smokers.