The purpose of this study is to investigate the alterations of kinetic characteristics in the step up/over test in patients with acute and chronic anterior cruciate ligament (ACL) deficiency. Twenty acute, twenty chronic ACL-deficient (ACLD) patients, and forty healthy controls participated in this study. The step up/over test was performed on a clinical force platform system to obtain the following variables: movement time, lift-up index and impact index. The percentages of the indexes from the stepping leg and time of step-up and swing- over were further calculated. These variables were examined statistically by using mixed repeated measure analysis of variance to reveal the differences between groups and between limbs. Smaller lift-up forces and longer swing-over time in both ACLD groups than those in the control group were found. One exception was the lift-up index in the affected side condition of the acute ACLD group, which was not significantly different from the controls. A reduced contribution to the impact index from the affected stepping leg was also found in the acute ACLD group. The acute ACLD group showed no significant change in their step-up strategy at the affected limbs, but they may have a poorer eccentric control of the affected knees on the step and thus put a larger portion of loading onto the unaffected landing leg. Our results indicated the lift-up index, swing-over time and load- distributing strategy at impact may be important parameters in monitoring functional recovery in patients with ACL deficiency. The detected alterations in these parameters could be used as a reference to design proper rehabilitation exercises for these patients.