Two hundred Aymara children (105 boys, 95 girls) born at high altitude (3,600 to 4,000 m; mean ± SD age, 9.5 ± 3.6 years; range, 1 to 17 years) were included in the study. Both parents of all children had typical Aymara surnames and self-identified themselves as Aymaras. We also studied 77 children of European ancestry (42 boys, 35 girls) born or having lived for at least 2 years at high altitude (age, 9.7 ± 3.7 years; range, 2 to 16 years); 8 of them were born in La Paz, and the others had lived there for 5.0 ± 3.6 years (range, 2 to 14 years). In addition, we studied 29 white children (16 boys, 13 girls) born and living at low altitude (450 m; age, 8.8 ± 2.6 years; range, 4 to 13 years). All children were healthy and free of physical or psychological infirmity. The Aymara children were studied either in El Alto (n = 146), a suburb of La Paz located at 4,000 m, or at the Instituto Boliviano de Biologia de Altura (n = 54) in La Paz (3,600 m). The white children living at high altitude were all studied in La Paz, and those living at low altitude were examined at the University Hospital in Bern, Switzerland (450 m). The experimental protocol was approved by the institutional review board on human investigation of the University of San Andres, La Paz, Bolivia, and the University of Lausanne, Switzerland. The parents gave written informed consent, and one of them was present during the examination.