Objective: The purposes of this study were to calculate and validate two models to estimate maximal oxygen uptake (VO 2 max) in Portuguese youths, aged 10-18 years, using a 20-meter shuttle run test (SR). Design: Subjects (54 girls and 60 boys) were divided into estimation ( n = 91) and cross-validation ( n =23) groups, and their VO 2 max was directly measured by wearing a portable gas analyzer during the SR. The Multiple Linear Regression (MLR) and Artificial Neural Network (ANN) tests were carried out considering sex, age, height, weight, body mass index (BMI) and SR stage as predictors of VO 2 max. Estimations from MLR and ANN were compared with three other previously published equations. Results: In summary, the equation estimated by MLR is more appropriate for Portuguese youths than the equation estimate by ANN or the other three previously-published equations (validation coefficient for the MLR model: r =0.84, P 0.05; SEE=4.9). Conclusion : For Portuguese youths, the following equation would be recommended: VO 2 max= 43.313 + 4.567*sex - 0.560*BMI + 2.785*stage. However, findings from this study also warn researchers that the use of equations to estimate VO 2 max may not be sensitive enough to detect small changes in individuals’ cardiorespiratory fitness in longitudinal observations and intervention studies, according to the dispersions of random error for all equations.