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The chest width and chest depth of 171 male and 180 female youths (9-19 yr) of European ancestry who were born and raised at high altitudes are compared with those of a sample of highland Aymara children. There is much less sexual dimorphism in chest size and shape among the Aymara than among the European children. This finding is probably at least partially due to altitude independent genetic differences but may also involve adaptation to altitude. After controlling for the greater stature of the Europeans, both the male and the female Aymara had significantly larger and deeper chests than their European counterparts. However, the slopes of the regression relationships between each chest dimension and stature did not differ significantly between ethnic groups, suggesting that the relative effect of hypoxia on chest growth is similar in both groups after an age of 9 years. Finally, comparison of the European highlanders with Aymara, Quechua and Tibetan highlanders indicates that a wide range of variation in chest dimensions relative to stature, and presumably in the underlying pulmonary system, is compatible with normal life at high altitude. This suggests that not all groups adapt to hypoxia with the same accelerated development of cardiorespiratory-system organs as is found in Quechua highlanders.