OBJECTIVE The objectives of this study were to investigate variations in bone mineral density values in 45-year-old women from a community sample in the City of Rio de Janeiro, in the same-name Brazilian state, and to compare them with US norms and international standards. METHODS In this cohort study, the age of 45 years is significant for the design, since clinical observation in this community indicates that it is approximately 5 years before menopause can be confirmed, thus preceding the typical postmenopause acceleration of the rate of bone loss. Nine hundred fifty-nine such women volunteered to participate in the investigation, conducted in 2 Rio de Janeiro teaching hospitals. Informed of procedures, potential risks, and benefits, they were screened for the inclusion criteria: (a) being 45-years-old: (b) being healthy (without obvious or diagnosed systemic disease, metabolic disease, endocrine disease, liver disease, cardiac disease, infectious disease, pulmonary disease, neurologic disease, dermatologic disease, inflammatory bowel disease, kidney disease, hereditary, or congenital conditions); (c) having regular and intact menstrual cycles; (d) having had normal healthy development to date; (e) having had high protein intake from birth to date; (f) having at least completed high school; (g) living in a dwelling equipped with running water, electricity, and public sewer; (h) being nonsmoking and non-drug abusing. Racial distinction was not among the inclusion criteria. After further informed consent, the 146 women who met all inclusion criteria had bone mineral density quantified--using the dual energy x-ray absorptionmetry method--and compared with US-born density norms for L2-L4 and the neck of the femur for young adults and the sample age group, which are endorsed by the World Health Organization and by the International Osteoporosis Foundation. RESULTS About three-quarters of the sample had normal bone mineral density values, 22.61% had osteopenia, and 2.73% had osteoporosis. CONCLUSIONS These findings, obtained from women whose regular and intact menstrual cycles demonstrated premenopausal hormonal levels, seem to attest to the importance of genetic predisposition, yet they warrant the authors' recommendation that interventions be instituted before age 45, specifically aimed at increasing the chances of all women, especially those genetically predisposed, of avoiding osteoporosis and its deleterious consequences.