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Maximal oxygen uptake (V02 max), generally accepted as a valid method for measuring state and change of aerobic fitness, was repeatedly measured in 93 males and 107 females 5 times over a period of 8 years. A direct measurement was made using a treadmill running test with constant speed (8 km/hr) and increasing slope. Oxygen uptake was analyzed continuously by an open-circuit technique. The reproducibility of V02 max estimated from interperiod correlations resulted in high test-retest correlations of approximately 0.9 in both males and females. Inspection of the longitudinal data from the multiple-longitudinal design with four measurements in three cohorts did not reveal confounding effects, such as time of measurement effects, cohort effects, and drop-out effects. A comparison of the longitudinal data evaluated over four years with data from a comparable control group that was measured once during the four-year period also failed to show any testing effects. In 40% of the males and 50% of the females no leveling-off in V02 max could be demonstrated; that is, there was an increase of more than 150 ml in the last stage of running. A comparison of subjects who showed leveling-off with those who showed no leveling-off supports the idea that in the age range 12-23 years leveling-off is not a prerequisite for reaching a true Vo2 max. Repeated measurement of V02 max, using a maximal running test on a treadmill appears to be a reliable method to describe the individual development of aerobic fitness in males and females in the age range 12-23 years.