The purpose of this study was to compare peak cardiorespiratory parameters during treadmill and cycle ergometry in severely overweight youth. Twenty-one participants from the Committed to Kids Pediatric Weight Management program at the Louisiana State University Health Sciences Center volunteered. Participants completed peak treadmill and cycle ergometer tests on separate days. In order to examine reliability, six subjects completed a second treadmill test and seven subjects a second cycle test. Physical characteristics included the following: Age (yrs) 12.5 ± 2.8; Body weight (BW) (kg) 78.5 ± 27.0, Height (m) 1.56 ± 0.13; and % fat 42.8 ± 7.5. No statistical significant differences (p ≤ 0.05) were found between treadmill and cycle peak tests. Treadmill VO₂ peak (l·min^-1) averaged 1.57 ± 0.40 and cycle 1.46 ± 0.30 and VO₂ peak relative to BW 21.5 ± 4.1 and 20.3 ± 5.5 for treadmill and cycle ergometry, respectively. Therefore treadmill values were 7.0% and 5.6% higher than cycle values. In normal weight or children and adolescents at risk for overweight, treadmill values typically average from 7 to 12% higher than cycle values. Reliability of VO₂ peak as indicated by intraclass correlation coefficients ranged from 0.70 to 0.96 for a single or repeated tests. Intra individual variability averaged 0.5% for VO₂ peak (l·min^-1) during treadmill ergometry and 5.7% for cycle ergometry. Also, standard errors of measurement were low (40 to 90 ml min or 1.0 to 1.7 ml.kg^-1. min^-1) for the peak treadmill or cycle tests. In summary, our data suggest that both treadmill and cycle ergometry provide reliable methods for determining VO₂ peak in overweight youth.