Most estimates of osteoporosis in older U.S. adults have been based on its occurrence in white women, even though it is known to affect men and minority women. In the present study, we used dual-energy X-ray absorptiometry measurements of femoral bone mineral density (BMD) from the third National Health and Nutrition Examination Survey (NHANES III, 1988-1994) to estimate the overall scope of the disease in the older U.S. population. Specifically, we estimate prevalences of low femoral BMD in women 50 years and older and explore different approaches for defining low BMD in older men in that age range. Low BMD levels were defined in accordance with an approach proposed by an expert panel of the World Health Organization and used BMD data from 382 non-Hispanic white (NHW) men or 409 NHW women ages 20-29 years from the NHANES III dataset. For women, estimates indicate 13-18%, or 4-6 million, have osteoporosis (i.e., BMD > 2.5 standard deviations [SD] below the mean of young NHW women) and 37-50%, or 13-17 million, have osteopenia (BMD between 1 and 2.5 SD below the mean of young NHW women). For men, these numbers depend on the gender of the reference group used to define cutoff values. When based on male cutoffs, 3-6% (1-2-million) of men have osteoporosis and 28-47% (8-13 million) have osteopenia; when based on female cutoffs, 1-4% (280,000-1 million) have osteoporosis and 15-33% (4-9 million) have osteopenia. Most of the older U.S. adults with low femur BMD are women, but, regardless of which cutoffs are used, the number of men is substantial.