BACKGROUND Despite a consensus regarding the correlation of peroneal strength deficit with chronic ankle instability (CAI), there are conflicting reports in regards to peroneal strength as assessed by isokinetic dynamometer in patients with CAI. The purpose of this study was to evaluate the changes of isokinetic strength in patients with CAI compared to ankle sprain copers and normal individuals. METHODS Forty-two patients (CAI group) with chronic ankle instability who were scheduled for the modified Broström procedure met inclusion criteria. Thirty-one ankle sprain copers (ASC group) who were eligible at 6 months after acute injury and 30 controls were recruited. The muscle strength associated with four motions of the ankle were evaluated using isokinetic dynamometer. RESULTS Peak torque for inversion and eversion at 60°/s angular velocity were significantly lower in the CAI group compared to the ASC and control group (P=.004, P<.001, respectively). Deficit ratio of peak torque for eversion at 60°/s and 120°/s in the CAI group were 33.8% and 19.8%, respectively, which indicated significant side to side differences (both P<.001). The evertor/invertor strength ratio (0.59) for eversion at 60°/s was significantly lower in the CAI group (P<.001). CONCLUSIONS As compared to the ankle sprain copers and normal individuals, patients with chronic ankle instability who were scheduled for modified Broström procedure demonstrated a significant weakness of isokinetic peroneal strength. Isokinetic muscular assessment can provide the useful preoperative informations regarding functional ankle instability focusing on peroneal weakness.