We wished to study the relationship between modifiable and nonmodifiable factors that were correlated with osteoporosis using a national sample of women aged 50 years and older who have never been on hormone replacement therapy (HRT). We used a cross-sectional study design with a nationally representative sample with a detailed clinical examination and a home interview. Between 1988 and 1994, 1953 postmenopausal women who had never been on HRT, aged 50 years and older, were examined as part of the Third National Health and Nutrition Examination Survey (NHANES III). Mexican Americans and non-Hispanic blacks were oversampled to produce reliable estimates for these groups. Bone density measurements of four proximal femur sites were assessed by using x-ray absorptiometry (DEXA). A DEXA measurement at any single femur site indicated osteoporosis if it was >2.5 standard deviations (SD) below the reference mean of 20--29-year-old women. The study demonstrated that numerous factors, both modifiable and nonmodifiable, were significantly related to the prevalence of osteoporosis. The modifiable factors identified were participation in physical activity (three to five times per week) and body mass index (BMI). Nonmodifiable factors included age, race, and mother's history. With the aging population, osteoporosis is a growing concern for the medical community. It is suggested that educational strategies are needed to increase awareness of factors that contribute to maintaining bone health among postmenopausal women. Emphasis may be placed on maintaining regular physical activity.