Abstract. Peripheral quantitative computed tomography (pQCT) is useful to measure volumetric bone mineral density (vBMD) distinguishing trabecular from cortical bones as well as quantity of bone geometry. In the present study, we examined the effects of age, grip strength and smoking on vBMD, bone geometry and bone strength index (polar strength strain index (SSIp)), and then compared with the differences between female and male by employing pQCT in Japanese 252 female and 230 male subjects. Age was negatively correlated with vBMD, cortical area (Ct.Ar) and cortical thickness (Ct.Th) as well as SSIp in both sexes, and the correlation coefficients were higher in female, compared with those in male. Although age was correlated with endocortical circumferences (En.Le) in both sexes, periosteal circumferences (Ex.Le) were correlated with age only in male. Volumetric BMD, Ct.Ar, Ct.Th and SSIp were significantly lower in the group with vertebral fractures, although En.Le and Ex.Le were similar between subjects with and without vertebral fractures. Grip strength was positively correlated with vBMD, Ct.Ar, Ct.Th as well as SSIp. The extent of correlation was much higher in female, compared with that in male. Ct.vBMD, Ct.Ar, Ct.Th and SSIp, but not trabecular vBMD, were significantly lower in the group with high Brinkman index (number of cigarettes smoked per day) × (duration of smoking (years)) in female. These parameters were not significantly different between groups with high and low Brinkman index in male. In conclusion, the present study demonstrated that age, grip strength and smoking affected forearm vBMD, bone geometry and bone strength index by pQCT. These effects were greater in female, compared with those in male. useful to quantify geometric properties of long bone, because it can be used to estimate area and circumferences of total bone as well as cortical area (Ct.Ar) and cortical thickness (Ct.Th) [2]. Moreover, pQCT helps to estimate bone strength by calculating polar strength strain index (SSIp), which has recently been elaborated to predict bone strength noninvasively [3]. Bone size increases with aging [4]. Increased bone loss after menopause is associated with increased periosteal apposition rate [5]. Our previous study revealed that an excess and a deficiency of endogenous parathyroid hormone (PTH) affect bone geometry determined by pQCT [6]. These findings suggest that the evaluation of bone geometry change is useful to estimate the bone fragility.Riggs et al. recently reported in a population-based study that the cross-sectional area of the vertebrae and proximal femur increased in both sexes over life, although vBMD at those sites decreased by 39-55% and 34-46%, respectively, with greater decreases in female than in male [7]. On the other hand, the increased muscle strength elevates BMD [8][9][10][11], and smoking enhances the risk of fractures [12,13]. However, no reports have been available about the sex difference in the effects of muscle strength and smoking on vBMD and bone geometry by usin...