The purpose of this study was to validate an equation that has been used to predict peak oxygen uptake (VO2peak) and, if invalid, to develop a new equation predicting VO2peak from performance on a cycle ergometer test. Forty-five 9- and 15-year-old children underwent a VO2peak test and were randomized into developmental (DEV) and cross-validation (C-V) groups. The equation under validation, which requires knowledge of resting energy expenditure (REE), underestimated VO2peak (p < .05), but once adjusted with a new parameter calculated in DEV, it cross-validated well (rYY' = .98, SE = .18 L . min-1). The accuracy of a new prediction equation built in DEV, not using REE, was confirmed in C-V (rYY' = .98, SE = .17 L . min-1) and the slope and intercept were not different from the line of identity (p < .05). VO2peak in schoolchildren can be predicted with good accuracy from an equation based on the whole sample [VO2peak = -1.5986 + 0.0115 . (maximal power output) + 0.0109 . (mass) + 0.1313 . (gender) + 0.0085 . (maximal heart rate)].