This study estimated an association between cognitive impairment and bone mineral density (BMD), a surrogate marker for cumulative estrogen exposure. This cross-sectional study included 120 postmenopausal women. Based on neuropsychological tests, the women were classified into three groups: those with subjective memory impairment (SMI, n=40), amnestic mild cognitive impairment (n=50), and early Alzheimer disease (AD, n = 30). A complete medical history was obtained, as were the results of dual-energy x-ray absorptiometry and spine x-ray examination. BMD and the prevalence of major osteoporotic fractures were compared among the groups after adjustment for all potential confounding factors. In addition, correlations of cognitive status and BMD were tested. The SMI group was younger than either the AD or the amnestic mild cognitive impairment group. There were no significant differences among the groups with respect to other clinical characteristics. Overall, cognitive impairment was associated with lower BMD. The BMDs at both the lumbar spine and total hip were significantly lower in the AD group than in the SMI group. Regression analysis revealed that the Mini-Mental State Examination score was positively correlated and that the sum of boxes of Clinical Dementia Rating was negatively correlated with BMDs at both sites. Major osteoporotic fractures were less prevalent in the SMI group, but the difference was not statistically significant. Cognitive impairment is significantly associated with lower BMD in postmenopausal women. This finding suggests that cognitive aging is clearly multifactorial, but estrogen deficiency may be one of the contributing factors.