Maximal oxygen uptake (VO2max) was predicted from maximal power output (MPO) in a progressive cycle ergometer test. The subjects were 232 men and 303 women 15-28 years of age. The relationship between VO2max and MPO was: VO2max (1.min-1) = 0.16 + (0.0117 x MPO) (w). A correlation coefficient of r = 0.88 was found between MPO and VO2max. Test-retest reliability was evaluated by two procedures. Standard deviations of test-retest differences in MPO and VO2max using the same standardized procedure in 35 subjects, were 10% and 8%, respectively, and Pearson correlations between test and retest values were 0.95 and 0.96, respectively. When MPO of tests conducted at the schools was compared to a standardized test performed by a physiologist in 267 subjects, test-retest Pearson correlation was 0.82. A prediction model only including MPO and explaining 80% of the variability in VO2max, is suggested for use in healthy adolescents and young adults.