BACKGROUND: Assessment of aerobic capacity (VO2max) is one of the essential components in exercise prescription for health promotion and maintenance. This study was conducted to compare the validity of four exercise tests to assess aerobic capacity in young adult male subjects by comparing them to the values measured from maximal ergometer exercise test (criterion VO2max). METHODS: Twenty young healthy adult males (M+/-SD=21.1+/-1.5 years) volunteered as subjects. Criterion VO2max was calculated and printed out automatically by computerized analysis system of expired air samples collected during maximal ergometer exercise test. VO2max was estimated for each subject from heart rate at submaximal workloads on the cycle ergometer using the Astrand-Rhyming nomogram(A/R) and Fox protocol(FOX) and the computerized multiple extrapolation method (XTP). The score got from Havard step test(H/S) was used to compare the validity. The validity of the procedures was based on the evaluation of the predicted VO2max (from A/R, FOX, XTP, and H/S) versus the criterion VO2max via the calculation of constant error (CE=mean difference for predicted minus criterion VO2max), r value, standard error of the estimate[SEE=SD(1-r2)(1/2)], total error [TE=(sigma(predicted VO2max - criterion VO2max)(2)/n)(1/2)]. RESULTS: In relation to criterion VO2max, the XTP and A.R underpredicted (XTP: 40.8 ml/kg/min SD=4.1; A/R: 37.3 ml/kg/min SD=5.0) and the H/S and FOX overpredicted (H/S: 48.0 ml/kg/min SD=5.9: FOX 46.3 ml/kg/min SD=5.5). Dunnett post-hoc procedures revealed that there were significant (P<0.05) mean differences (CE) for VO2max from A/R versus criterion. The validity coefficients for VO2max derived from XTP, H/S, A/R, and FOX were 0.68, 0.53, 0.50, and 0.49, respectively. TE of the XTP, FOX, H/S and A/R, which accounts for the effects of both the CE and SEE, were 5.73, 6.13, 6.75, and 8.87, respectively. CONCLUSIONS: These results suggest that the XTP is recommended first for estimation VO2max in young adult males. It is also considered that further studies about female and other age groups are necessary.