This study examined the validity and reliability of maximal aerobic power (VO2max) during wheelchair ergometry (WE) in wheelchair athletes with cerebral palsy. Six class 3 and class 4 male athletes with cerebral palsy completed two graded exercise tests to volitional fatigue on a wheelchair ergometer mounted on frictionless rollers. Four athletes were also able to complete two tests of bicycle ergometry (BE). Although the reliability coefficients for the VO2max during the two exercise modes were high (.89 and .93 for the WE and BE tests, respectively), the validity coefficients for this variable (ie, the correlations between WE and BE) were poor (.31 and -.24 for trials 1 and 2, respectively). Examination of the individual data indicated that athletes who used wheelchairs as their primary mode of ambulation had higher VO2max values during WE; whereas, those who used canes or no aids for daily ambulation had higher values on BE. Because of the specificity of the, VO2max response, it is recommended that the primary mode of ambulation be considered when deciding on the testing mode for evaluating the cardiorespiratory fitness of athletes with cerebral palsy.