The aim of this study was to evaluate the alterations in eccentric evertor/concentric invertor strength ratio and their importance in the chronically unstable ankle. Eight patients with chronic ankle instability (CAI) and nine healthy individuals participated in this study. Isokinetic concentric and eccentric invertor and evertor muscle strength measurement was carried out at an angular velocity of 120°·sec^-1 by measuring maximal force moments (torque) during isokinetic ankle inversion and eversion movements. Functionally, evertor/invertor muscle strength ratios (E/I strength ratio) were calculated separately based on peak moment and angle-specific moments obtained at 0°, 5°, 10°, 15°, 20° ankle joint angles. Peak and angle-specific eccentric evertor strength values at 0°, 5°, 10°, 15°, 20° were significantly lower in the chronic ankle instability (CAI) group. In spite of this, no differences were obtained for peak and angle-specific concentric invertor torque values. Eccentric evertor/concentric invertor strength (E_ecc/I_con) ratios were also significantly lower in the CAI group, but only at 15° and 20°. Eccentric evertor muscle torque and end range (15°-20°) E_ecc/I_con strength ratio for the chronically unstable ankle were significantly different from those for the healthy ankle. For this reason, measurements of end range eccentric/concentric strength ratios are more valuable in monitoring chronic ankle injuries and rehabilitation should include not only concentric muscle strengthening but also eccentric muscle strengthening, particularly for the evertor muscles.