The aim of this study was to compare the validity of two field tests of aerobic fitness for predicting maximal oxygen uptake (VO2max) in a group of adolescent schoolchildren. Twenty four schoolboys (means age = 15.6 +/- 0.6 yr) and twenty four schoolgirls (means age = 15.4 +/- 0.7 yr) underwent three different tests of aerobic capacity, with one week separating each assessment. The test were: (a) VO2max, using a continuous, progressive treadmill protocol, (b) a physical work capacity test at a heart rate of 170 beats per minute (PWC170), and (c) a 20-metre endurance shuttle run (20-MST), using a modified lapscoring protocol. Heart rates were monitored continuously in all tests. Results (means +/- SD) for boys n = 23) and girls (n = 18) respectively were: VO2max, 53.3 +/- 5.9 and 42.6 +/- 5.8 ml.kg-1.min-1; PWC170, 2.84 +/- 0.47 and 1.86 +/- 0.39 w.kg-1; 20-MST, 81.7 +/- 15.9 and 50.4 +/- 12.5 laps. Linear regression of PWC170 and 20-MST on VO2max scores (n = 41), revealed similarly high powers of prediction for both field tests (PWC170 vs VO2max, r = 0.84; 20-MST vs VO2max, r = 0.87), with an indication that the shuttle run test may be preferable for use with girls. In conclusion, the PWC170 and 20-MST field tests both appear to be valid predictors of VO2max in adolescent schoolchildren. However constraints of time and tester expertise favour the 20-MST for the assessment of aerobic capacity in the field.