The number of obese children has dramatically increased and the physical fitness level of children has decreased during the past decade. Maximal graded exercise tests (GXT) may provide valuable information concerning the baseline aerobic fitness and the effectiveness of intervention strategies to improve aerobic fitness in children. The optimal type of ergometry to use while conducting maximal GXT testing children, specifically in lean versus obese children, has not been fully elucidated. PURPOSE To compare VO2 peak measures from maximal GXT using treadmill and bicycle ergometry, in lean (BMI <85th percentile for age and sex) versus obese (BMI >95th percentile for age and sex) prepubescent (Tanner Maturation Stage ± 2) children ages 7 to 11 yrs. METHODS Maximal GXT were performed to determine VO2 peak measures using treadmill (TM) and bicycle (BK) ergometry, and body composition was determined using total body dual energy x-ray absorptiometry (DXA) scans in lean children (LC; n = 21) and obese children (OC; n = 20). Group mean VO2 peak comparisons were determined using two-way ANOVA with repeated measures. RESULTS Absolute peak VO2 (L) did not significantly differ between the lean and obese children for the TM GXT (1.21 ± 0.11 LC vs 1.49 ± 0.11 OC) or for the BK GXT (1.34 ± 0.08 LC vs 1.46 ± 0.08 OC). The LC had significantly greater (p <0.05) peak VO2 when expressed relative (ml/kg) to body weight for the TM GXT (37.0 ± 2.0 LC vs 23.4 ± 2.1 OC) and BK GXT (39.0 ± 1.7 LC vs 25.4 ± 1.8 OC), and for the BK GXT when expressed relative to fat-free mass (FFM; 50.2 ± 1.8 LC vs 42.6 ± 1.8 OC) but not for the TM GXT (44.2 ± 2.6 LC vs 43.0 ± 2.6 OC). The LC had significantly greater absolute peak VO2 for the Bike vs Tmill GXT (1.34 ± 0.08 BK vs 1.21 ± 0.11 TM) and significantly greater peak VO2 when expressed relative to FFM (50.2 ± 1.8 BK vs 44.2 ± 2.6 TM), but not when peak VO2 was expressed relative to body weight (39.0 ± 1.7 BK vs 37.0 ± 2.04 TM). The obese children had no significant differences between Bike and Tmill GXT measures when the peak VO2 was expressed as an absolute (1.46 ± 0.08 BK vs 1.49 ± 0.11 TM), relative to body weight (25.4 ± 1.8 BK vs 23.4 ± 2.1 TM), or relative to FFM (42.6 ± 1.8 BK vs 43.0 ± 2.6 TM). CONCLUSION Obesity status (lean vs obese) should be considered during the selection of ergometry type used during maximal GXT in prepubescent children. Supported by University of Kentucky Graduate Research Student Support Fund and the University of Kentucky Pediatric Exercise Physiology Laboratory