PURPOSE: To compare muscle strength, muscle endurance, and postural stability in both the affected and unaffected ankles between patients with chronic ankle instability (CAI) who underwent conservative treatment and those who underwent the modified Broström procedure (MBP). METHODS: A total of 67 patients (37, conservative treatment; 30, MBP) participated. Muscle strength and muscle endurance were measured using an isokinetic device, and postural stability was tested using a postural stabilometry system. We used the independent t test for continuous variables with a normal distribution and Fisher's exact test for categorical variables. RESULTS: There was no difference in the muscle strengths of the affected and unaffected ankles between the groups. The muscle endurance of plantarflexion and inversion muscles was significantly lower in the affected ankles of the conservative treatment group than in those of the MBP group (plantarflexion: 209 ± 103.1 vs. 318 ± 162.2, p = 0.001; inversion: 93 ± 58.7 vs. 154 ± 65.9, p < 0.001). Static postural stability testing showed no significant differences between the affected and unaffected ankles of the two groups. In the dynamic postural stability test, the overall, anterior-posterior, and medial-lateral stability indices were all significantly higher in the affected ankles of the conservative treatment group than in those of the MBP group (p < 0.001, p = 0.004, p = 0.004, respectively), with no differences observed in the unaffected ankles. CONCLUSION: The MBP may significantly improve muscle endurance and dynamic postural stability in CAI patients in whom conservative treatment has failed. Therefore, clinicians should consider using MBP instead of conservative treatment when patients show severe muscle weakness or dynamic postural instability. LEVEL OF EVIDENCE: Case-control study, Level III.