AbstractTreadmill measurements of maximum oxygen intake were made in 218 boys and 136 girls 6–16 years of age who were classified as nutritionally normal or marginally malnourished. max (1· min−1)increased with age, was lower in the undernourished than in the normal, and was significantly lower in girls than in boys. The sex differences persisted when max was expressed in terms of body weight and lean body mass, indicating that differences in physical activity between sexes may be the cause. The lower values of absolute max have implications for the productive ability of these children in heavy physical work when they are adults. When expressed in terms of body weight there are no major differences between Colombian and European children in the magnitude of their max va