It has been postulated that coactivation of antagonist hamstring musculature during active knee extension aids the anterior cruciate ligament (ACL) in maintaining joint stability by exerting an opposing torque to anterior tibial displacement induced by the quadriceps. It was the purpose of this study to compare contralateral patterns of hamstring coactivation in subjects who have suffered ACL dysfunction with subjects who have normal knees. Five subjects who had suffered ACL dysfunction (INJ) and five uninjured (UNI) subjects performed maximal flexions and extensions of the knee on a modified isokinetic dynamometer at 100 degrees and 300 degrees.s-1. Simultaneous recordings of torque, angular displacement, and electromyographic (EMG) activity from the hamstring musculature were computer processed. No significant differences (P < 0.05) in torque were found between ACL/normal and right/left limbs for the INJ and UNI groups, respectively. For the INJ subjects, mean coactivation of the normal limb hamstrings during extension averaged twice that of the ACL limb. In contrast, no significant difference was found for the same comparison between right and left limbs in the UNI group. The results of this study suggest that asymmetry in hamstring coactivation during knee extension may result from ACL dysfunction.