The Bruce treadmill protocol was used to evaluate the exercise endurance in a pediatric urban population. The sample population consisted of 525 healthy children 4 to 18 years of age (303 males and 222 females). Endurance time increased with age in males and increased up to 10 to 12 years of age in females, thereafter leveling off. Results were compared with those of a widely used standard reference of normal for treadmill exercise testing in children. Mean endurance times were significantly shorter compared to those of the reference study (p < 0.03). When the subjects were grouped in percentiles based on the reference study, 61% of males and 81% of females performed below the 25th percentile. There was a strong negative correlation between body mass index and endurance time (p < 0.0001), suggesting that obesity is a major contributor to decreased physical fitness. Compared to subjects in the reference study, children from a large U.S. metropolitan area have markedly diminished exercise endurance times, suggesting poor physical fitness. Inactivity with consequent lack of fitness, if unchecked, are likely to lead to cardiovascular disorders in adulthood. Efforts should be made to encourage physical fitness in children.