Objective: The present study was designed to determine if low pulse oxygen saturation (Spo2) in post-menopausal women at high altitude was related to high serum testosterone/estradiol (T/E2) ratio. Method: Studies were carried out in 191 women living in Cerro de Pasco, Peru (4340 m above sea level) and in 56 women living in Lima (150 m asl). Body weight, height, Spo2, hematocrit, serum follicle stimulating hormone (FSH), T and E2 levels were measured in all women. Body mass index (BMI), menopausal status and serum T/E2 ratio were calculated from data. Result: At high altitude, the values of Spo2, serum T, and serum E2 were significantly lower in post-menopausal than in pre-menopausal women. Hematocrit, serum FSH levels, and serum T/E2 ratios were significantly higher in post-menopausal women. Serum T and E2 levels, T/E2 ratio, Spo2, and hematocrit levels were not further changed with time after menopause. Women with Spo2 < 85% had lower serum E2, a higher serum T/E2 ratio, and higher hematocrit values than women with Spo2 > 90%. At high altitude, multiple regression analysis showed that low Spo2 was related to menopausal status (-2.6 ± 0.83; β ± S.E.; P < 0.002) and not to chronological age (-0.06 ± 0.04; P: NS). Further analysis showed that low Spo2 was related to high basal serum FSH levels and a high serum T/E2 ratio in the presence of an interaction between FSH and T/E2. Conclusion: Low values of Spo2 in women at high altitude were related to a high T/E2 ratio. © 2000 International Federation of Gynecology and Obstetrics.