Strengthening of the evertor muscles is widely advocated as a key component of lateral ankle sprain rehabilitation, but our clinical observation of impaired invertor muscle performance among many patients suggested the need for this study of isokinetic performance deficiencies. Subjects were 30 physically active adolescents, ages 14-19 years, who had recently sustained a lateral ankle sprain or who had symptoms of chronic lateral ankle instability. Eversion/inversion testing was performed on a Biodex isokinetic dynamometer at speeds of 30 and 120 degrees/sec. Analysis of variance results demonstrated significantly greater (p < .05) invertor deficits than evertor deficits for both peak torque and average power at both tests speeds. The findings of this study suggest that a lateral ankle ligament injury may be associated with an invertor muscle performance deficiency, and that restoration of a normal evertor/invertor strength relationship may be accomplished through performance of an isotonic ankle strengthening program.