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Human fetal growth, reflected in birth weight and linear dimensions of the newborn, is decreased at high altitude although the causal role of hypoxia remains presumptive. In an effort to isolate the source of fetal growth deficit in high-altitude newborns, we examined newborn body composition in a sample which included 175 infants from high altitude and 71 from low altitude in Bolivia. After controlling for variation associated with sex of the infant, ethnic group, parity and maternal size, birth weight was reduced 339 grams at high altitude and crown-heel length was reduced 1 cm, but the sum of five skinfolds was almost 5 mm greater. Newborn brachial muscle area did not vary by altitude. These results indicate that fetal undemutrition is not responsible for the reduced newborn dimensions observed at high altitude and further suggest that hypoxia may exert a direct effect on human fetal growth.