Longitudinal studies of adolescent males have shown that absolute maximal aerobic power (V̇O2max) and distance-running performance improve with age, while V̇O2max expressed relative to body mass remains stable. These earlier studies used subjects that were engaged in distance-run training; therefore, it was not possible to assess the relative importance of growth and/or run training to improved performance. The purpose of this study, therefore, was to quantify longitudinal changes in V̇O2max, running economy (RE), and distance-running performance in non-run-trained young males. Six subjects were tested at mean ages of 9.9 (T1) and 16.8 years (T2). Statistical tests of mean values revealed that over the 7-year period, relative V̇O2max remained unchanged (T1, 48.9 ml·kg-1·min-1; T2, 47.8 ml·kg-1·min-1) and RE improved (T1, 234.2; T2, 202.8ml·kg-1·min-1), 9-min run distance increased (T1, 1637 m; T2, 2115 m), and the estimated percent of V̇O2max incurred during the 9-min run increased from 85.8% (T1) to 99.5% (T2). It was concluded that the improvement in distance-running performance observed in adolescent boys was not dependent upon run training. In view of the fact that maximal aerobic power remained stable, this change appears to be attributable to better running economy and an ability in runs of a fixed duration to perform at a higher relative work load.