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Reproduction among Japanese women living at high altitudes in Bolivia was examined in comparison with their counterparts in the Bolivian lowlands. While there were no differences in age, contraceptive usage and the incidence of fetal deaths between the groups, both pregnancies and livebirths were significantly fewer in the women at high altitudes than in their lowland counterparts. However, multiple regression analyses controlling for age, age at first pregnancy (age at marriage) and age at menarche indicate that the effect of altitude on reducing fertility has not been as great as would be expected from the literature. Altitude explained less than 3% of the variance in the numbers of pregnancies and of livebirths; the difference in reproductive performance was largely attributable to later marriage in the high-altitude women than in their low-altitude counterparts. The mean birth weights of infants conceived and born at high altitudes (2673 g) were significantly smaller than those bom at low altitudes (3095 g). While the mean birth weights of Japanese newborns appear to be lower at either altitude than those of Bolivian or European infants, the difference of 422 g between the high- and low-altitude infants was within the range of difference reported in previous studies for Andean and Himalayan natives, suggesting that no ethnic differences exist in the effect of hypoxia on reducing birth weight at high altitude.