We measured bone mineral density (BMD in g/cm(2)) of the spine (L2-L4) and femur (four regions) in 1472 and 1487 cases, respectively, of ambulatory white women ages 20-79 years in the USA. A DPX densitometer was used in a mobile setting. The BMD values for women up to 69 years corresponded closely with published values for the USA, the UK, and northern Europe; our values were somewhat lower than those from other studies only in women over 70 years. The USA data were combined with data from Europe to give reference curves on about 12,000 subjects. Decreases of BMD with age in women below 50 years were much smaller than in older women (0.2% versus 0.6-1.0% per year). Femoral bone decreased from the neck region, but not the trochanter with age; the decrease of total femur BMD with age was due to loss from the former region. Loss of bone mineral content (BMC in g) from the femur neck and total femur region did not accelerate until after age 50 years, much like the spine. The apparent decrease of BMD in these regions that begins about age 40 actually is due to an increase of bone area. About 20% of USA women aged 50-79 years had BMD levels for the lumbar spine, or for the femur neck, more than -2.5 SD below the average values in young adult women 20-39 years old. Body weight had several times more impact on BMD than height, and in fact, a change of 1 kg in postmenopausal women was commensurate with the effect of a 1-year change in age. Subjects in the lowest quartile of body weight had T-scores that were 1 SD below those in the highest quartile.