A dual-energy X-ray absorptiometry (DXA) machine was used to measure the bone mineral density (BMD) of both femora in 760 female volunteers. Each volunteer completed a questionnaire and exclusion criteria were applied such that only 480 of these were considered normal subjects. The remaining 280 women failed to comply with the criteria and were considered 'abnormal'; their BMD results were analysed separately. Two abnormal subgroups, one with previous long bone fractures and one with radiologically diagnosed osteopenia, were studied. BMD values for femoral neck, Ward's triangle and trochanter were compared between the two femora in all the above groups. No dominance relationship was found when comparing left to right femur, averaged over any population studied, but large differences were found between the femora in individual volunteers. There was a high correlation between BMD in opposing femora of 0.91, 0.91 and 0.84 for the femoral neck, Ward's triangle and trochanter respectively. However, in normal subjects the percentage variation in these regions ranged up to 34%, 64% and 80% respectively at the different femoral sites. In addition, the normal population was divided into two subgroups, one in which the density difference between the femora was large, and the other in which the difference was statistically insignificant. The analytical and anatomical variations between these two groups were investigated. Only part of the difference appeared to be due to analytical problems and it seems that there is a genuine difference in femoral density. Poor correlation for femoral neck percentage density difference was found with average BMD, age, height and weight in the normal population.(ABSTRACT TRUNCATED AT 250 WORDS)