Data on the number of U.S. women with low femoral bone mineral density (BMD) are currently available only from indirect estimates. We used dual-energy X-ray absorptiometry (DXA) measurements of femoral BMD from phase 1 of the third National Health and Nutrition Examination Survey (NHANES III, 1988-1991) to estimate prevalences of low femoral BMD in women ages 50 years and older using an approach proposed recently by an expert panel of the World Health Organization (WHO). Cutpoints for low BMD were derived from BMD data of 194 non-Hispanic white (NHW) women aged 20-29 years from the NHANES III dataset. The prevalence of older U.S. women with femoral osteopenia (BMD between 1 standard deviation [SD] and 2.5 SD below the mean of young NHW women) ranged from 34-50% in four different femur regions, which corresponds to approximately 12-17 million women. The prevalence with osteoporosis (BMD > 2.5 SD below the mean of young NHW women) ranged from 17-20%, or approximately 6-7 million women. Prevalences were 1.3-2.4 times higher in NHW women than non-Hispanic black women (NHB), and 0.8-1.2 times higher in NHW versus Mexican American (MA) women. The estimated numbers of NHW, NHB, and MA women with osteopenia were 10-15 million, 800,000-1.2 million, and 300,000-400,000, respectively; corresponding figures for osteoporosis were 5-6 million, 200,000-300,000, and 100,000 respectively. Thus, the first data on BMD from a nationally representative sample of older women show a substantial number with low femoral BMD.(ABSTRACT TRUNCATED AT 250 WORDS)