The purpose of the study was to determine the impact of nutrition and physical activity on growth and health of 228 native Bolivian prepubertal girls and boys 10-12 years of age, from high socioeconomic status (HSES) and low socioeconomic status (LSES), living at high altitude (HA) in La Paz (∼4,000 m) and at low altitude (LA) in St. Cruz (∼400 m). In the 4 years of the study, dietary information and daily physical activities were obtained with the 24-hour recall method by interviewing the child and mother. All food items were converted into grams, and nutrients were calculated by using food composition tables for Latin America. Physical activities were estimated in four intensity levels and calculated in terms of energy output. The results show that nutritional intake is influenced by socioeconomic status, but not by altitude. Overall, the energy and nutrient intake of the HSES girls and boys was significantly higher (P ≤ 0.01) than the intake of the LSES children at both altitudes; no sex effect was evident. HSES girls and boys consumed greater amounts of nutrients, especially protein and fat; they were taller but also fatter. The physical activity of LSES children was higher than that of HSES children. The lower energy intake and the higher energy expenditure of LSES children were reflected in their smaller body size. Am. J. Hum. Biol. 9:51-62 © 1997 Wiley-Liss, Inc.