This study was conducted to establish reference values for percent oxygen saturation of hemoglobin (SaO2, %) and heart rate (HR, bpm) in children living at high altitude (4,100 m) and to relate possible differences in the variables with ethnic origin. Healthy children from a mine-located school (Tintaya, n = 417), a nearby school (Marquiri, n = 474), and a rural Andean community (Nuñoa, n = 373) were investigated. The samples included different ethnic combinations, with the Nuñoa children having a predominant Quechua ancestry. Mean SaO2 for all ages was substantially lower in all high altitude children compared to values considered normal for sea level. Among the three samples, SaO2 was higher (91.3 ± 2.7) and HR was lower (84.8 ± 13.6) in Nuñoa than in Tintaya (SaO2, 89.8 ± 2.5; HR, 91.7 ± 14.9) and Marquiri (SaO2, 89.6 ± 3.1; HR, 88.5 ± 12.9) (P < 0.05). There was no sex difference and only a weak age-dependent trend for SaO2. Values considered abnormal at sea level were observed in all healthy high-altitude children. Higher SaO2 and lower HR in Nuñoa children may suggest a better degree of acclimatization to altitude. © 2001 Wiley-Liss, Inc.