The aging of Brazilian population has been well-documented. A longer time since menopause, poor self-perception of health, and having arthrosis and balance problems are factors associated with postmenopausal osteoporosis. Poor self-perception of health, menopausal treatment with natural remedies, smoking, and decreased functional capacity are associated with early onset of the disease. Purpose: This study aims to evaluate the prevalence of osteoporosis and its associated factors in Brazilian women over 50 years of age and to obtain information on factors related to the early onset of the disease. Methods: A cross-sectional study with 622 women over 50 years of age residing in Campinas/Brazil was conducted between May 10 and October 31, 2011 in the form of a population survey. A questionnaire was applied by trained interviewers. Osteoporosis was classified either as self-reported or as osteoporosis diagnosed by bone densitometry. Statistical analysis was carried out by chi-square test, Poisson regression analysis, and Cox multiple regression model. Results: The mean age of the women was 64.1 years. The prevalence of self-reported osteoporosis was 21.3 %. A longer time since menopause (prevalence ratios (PR), 1.04; 95 % CI, 1.03-1.05; p < 0.001); self-perception of health as fair/poor/very poor (PR, 1.73; 95 % CI, 1.29-2.33; p < 0.001); having arthrosis (PR, 1.83; 95 % CI, 1.30-2.59; p < 0.002) and having problems maintaining balance when taking a bath or going down stairs (PR, 1.52; 95 % CI, 1.07-2.14; p = 0.020) were associated with osteoporosis. The variables associated with early onset of the disease were: self-perception of health as fair/poor/very poor (coefficient, 0.77; p < 0.001), menopausal treatment with natural remedies (coefficient, 1.01; p < 0.001), smoking or having smoked >20 cigarettes/day (coefficient, 1.02; p = 0.003), and problems in running/lifting something heavy/practicing sports/doing heavy work (coefficient, 0.60; p = 0.029). 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