The purpose was to investigate whether deviations in gait parameters or muscular activity patterns can be detected in the injured and healthy leg of chronic ACL-deficient subjects. Sixteen medium-level active chronic ACL-deficient patients classified as "copers" (injury duration: 12-240 months, age 17-52 years) and 15 healthy subjects (age 20-33 years) walked at self-selected speed along a 10-m runway with a level force-plate. Gait specific data, ground reaction forces, knee and ankle angles, and EMG were documented. Knee laxity was increased and the functional scores (Lysholm, KOOS) decreased in the ACL- deficient subjects, whereas the Tegner activity level score was normal. Gait speed, stride length and stance time did not differ between ACL-deficient subjects and controls. Ground reaction forces (magnitude and times), as well as knee and ankle angles at selected points during stance and swing phases were normal in the ACL-deficient subjects compared to controls. The total duration of m. tibialis anterior (TA) activity was longer in ACL-deficient subjects than in controls (ACL-deficient injured leg vs. controls, P < 0.05). In addition, the onset of lateral gastrocnemius (LG) muscle activity occurred earlier in ACL-deficient patients (P < 0.03), resulting in a TA-LG cocontraction in the ACL-deficient but not in the control group. In conclusion, chronic, medium-level active ACL-deficient patients showed abnormalities in muscular activity patterns during gait compared to control subjects, whereas there were no detectable changes in ground reaction forces and 3D kinematic data. As the aberrant muscular activity pattern may be of importance for an even gait, it is proposed that EMG recordings may give additional information in the evaluation and rehabilitation of gait when the ACL is absent.