BACKGROUND: At the moment the studies lead at world-wide level and even in our country have thrown discrepant results about the relation between osteoporosis, parity and age of menarche. OBJECTIVE: To investigate the relation of osteoporosis in postmenopausal Mexican women with multiparity and age of menarche. PATIENTS AND METHOD: A retrospective and analytical cross-sectional study, with a non-probabilistic sampling technique in women rightful claimants of the IMSS, San Luis Potosi. In all of them the bone mineral density was measured with X-ray dual absorptiometry in the distal forearm. Reproductive history and age of menarche were obtained by the addition of these items to the previously validated Albrand questionnaire. Women were divided into groups according to the number of pregnancies in: normal parity (0 to 3 childbirths) conformed by 112 patients (46%) and multiparity (> or = 4 pregnancies), 131 women (54%). In relation to menarche with an average of 12.98 years, from this number we divided them in: early menarche (< 13 yrs) and late menarche (> or = 13 yrs). RESULTS: 243 women were studied, with an average of age of 55.92, rank 31 to 80 years. Using the criteria of the World Health Organization, 18% of postmenopausal women had osteoporosis, 39% had osteopenia and 43% had bone normality. No association was found between the number of pregnancies and osteoporosis. Additionally we observed that the women who had four or more children were older than the other women, average 57.42 against 54.16. Also there was significant negative correlation (r = -0.43) between age and densitometry. We found that an age greater to 13 years in the appearance of the menarche was related to osteoporosis (OR 4.46, p: 0.035). CONCLUSIONS: In postmenopausal women a menarche older than 13 years is a risk factor for osteoporosis.