Patellofemoral pain syndrome is a common condition athletes suffer from. One of the predictive factors of this syndrome is reduced activity ratio of vastus medialis oblique (VMO) to vastus lateralis (VL). The aim of this descriptive study was to compare the electromyographic activity ratio of vastus medialis oblique to vastus lateralis during maximum voluntary isometric contraction of quadratus femoris on Biodex between athletes with and without patellofemoral pain syndrome. For this purpose, 16 national team male athletes (volleyball, handball and taekwondo) with patellofemoral pain syndrome aged between 18 and 30 and 16 healthy male athletes voluntarily participated in this study. EMG activity of VMO and VL muscles were recorded by surface electrodes and telemetric EMG system on Biodex at 15, 30 and 45 degrees of knee flexion and VMO: VL EMG activity ratio was calculated. To compare muscle activity ratio between the two groups, t test was used. The results showed no significant difference in the electrical activity of VMO and VL muscles in knee flexion angles between athletes with and without patellofemorl pain syndrome (P>0.05). The EMG ratio of VMO:VL was higher than 1 in both groups, but this ratio was higher in healthy athletes. Finally, it seems that sporting activities as a preventive factor impede disturbance of electrical activity of VMO that prevent reduction of VMO: VL ratio.