It has been argued that motivation significantly affects the measurement of aerobic capacity when using field tests with children. In this study, the impact of generalized self-efficacy on performance (Stage Completed) in the Léger shuttle run is examined in a cohort of children (N = 2,245, 9.38 ± 0.52 years old) in Grade 4 from 75 elementary schools. Children completed the Children's Self-perceptions of Adequacy in and Predilection for Physical Activity scale (CSAPPA) to establish levels of generalized self-efficacy toward physical activity, were measured for height and weight, and then completed the Léger Shuttle run to predict aerobic capacity. Regression analysis was used to study the impact of self-efficacy on test performance. After adjusting for age, gender, and BMI, two of the three CSAPPA factor subscales, higher perceived adequacy regarding physical activity (β = 0.196, P < 0.001) and greater predilection to select physical over sedentary activities (β = 0.123, P < 0.001), were independently associated with better test performance as indicated by stage completed. Together, self-efficacy accounted for 9% of the total variation in Léger shuttle run performance. A significant interaction between BMI and perceived adequacy was found (β = −0.106, P < 0.005). Children with both high BMI scores and below average perceived adequacy had the poorest performance results. Generalized self-efficacy, as measured by the CSAPPA, is significantly related to Léger shuttle run performance. Moreover, self-efficacy influences the relationship between other known factors affecting test performance (BMI), suggesting that self-perception of ability/competence has a complex effect on test performance. These results illustrate the importance of considering psychological factors when interpreting physiologic assessments in children. Am. J. Hum. Biol., 2008. © 2007 Wiley-Liss, Inc.