OBJECTIVE The metabolic energy cost of walking is altered by pathological changes in gait. It is thought that anterior cruciate ligament (ACL) deficiency alters the energy requirement for level walking through its effect on gait pattern. In this study, it is hypothesised that the metabolic energy cost of walking would improve after ACL reconstruction. METHODS Eight patients who were undergoing ACL reconstruction for an isolated rupture were included in this prospective study. Clinical examinations, Lysholm scores and metabolic tests were performed preoperatively and at 3, 6 and 12 months after ACL reconstruction using autologous quadruple hamstring tendons. For the metabolic evaluation, net oxygen cost was calculated while walking on a treadmill at 50-, 70- and 90-m/min velocities. A two-way factorial ANOVA was performed in order to evaluate the primary effects and interactions of the time point and velocity variables on net oxygen cost. RESULTS All patients had positive Lachman and anterior drawer tests preoperatively that became negative postoperatively and remained negative until the last follow-up point. The mean preoperative Lysholm score was 66, whereas the mean postoperative follow-up scores were 85, 91 and 94, respectively. The interaction between follow-up time point and velocity was not significant. Regardless of the selected velocity, the net oxygen cost was lower than that at preoperative levels at each postoperative time point (p < 0.05). CONCLUSIONS The results of the present study indicate that the energy cost of level walking in chronic ACL-deficient patients improves after ACL reconstruction. Cause-effect-based studies with correlation evaluations that compare kinetic, kinematic and electromyographic data and metabolic cost calculations should facilitate more accurate analyses. METHODS Therapeutic study, Level 4.