Although there is a tendency toward gait normalization after anterior cruciate ligament reconstruction, altered moments about the knee flexion-extension axis have been reported. It is possible that these gait alterations relate to donor site morbidity associated with the graft harvest.There is a relationship between graft type and external knee moments during walking.Controlled laboratory study.Three groups were compared: 17 patellar tendon anterior cruciate ligament reconstruction patients (mean, 11 months after surgery), 17 hamstring tendon anterior cruciate ligament reconstruction patients (mean, 9.3 months after surgery), and 17 matched controls. A 3-dimensional motion analysis and force plate system was used to determine sagittal plane kinematics and kinetics of the lower limb during comfortable-speed walking.There were significant differences in the moments about the knee that related to graft type. The external knee flexion moment at midstance was significantly smaller than that in the control knees in 65% of patients in the patellar tendon group and 29% of patients in the hamstring tendon group. In contrast, the external knee extension moment at terminal stance was significantly smaller than that in the control knees in 53% of subjects in the hamstring tendon group and 23% of subjects in the patellar tendon group.There are graft-specific differences in knee biomechanics after anterior cruciate ligament reconstruction that appear to relate to the donor site.Considerable debate continues as to whether the patellar tendon or the hamstring tendon graft is preferable for anterior cruciate ligament reconstruction. It is therefore clinically relevant to understand the biomechanical differences in knee function associated with both graft types.