This study was undertaken to determine the effect of distal hamstring lengthening (DHL) on hip and knee sagittal kinematics, and to investigate the validity of modeled hamstring length for clinical use. Patient group consisted of 28 patients (56 limbs, mean age 7.4 years) with spastic diplegia who underwent bilateral DHL and tendo-Achilles lengthening with/without rectus femoris transfer (RFT) (DHL+RFT subgroup, 40 limbs; DHL subgroup, 16 limbs). Kinematic data was obtained by gait analysis, and hamstring lengths were obtained using a musculoskeletal modeling technique. Postoperatively, knee extension improved (p<0.001) without aggravating anterior pelvic tilt (p=0.565). However, DHL aggravated anterior pelvic tilt in the DHL subgroup (2.2 degrees, p=0.011). In terms of concurrent validity, hamstring length was found to be correlated with mean pelvic tilt (r=0.798, p<0.001) and popliteal angle (r=-0.425, p=0.001), but the correlation between hamstring length and knee flexion at initial contact was minimal (r=0.068, p=0.753). In terms of construct validity, DHL did not lengthen mean hamstring length (p=0.918). In conclusion, DHL appeared to significantly improve knee motion in patients with spastic diplegia. Furthermore, DHL did not increase pelvic tilt, when performed with RFT. Modeled hamstring length is believed to have limited validity in patients with cerebral palsy, because it does not reflect knee kinematics or postoperative change when DHL was combined with multilevel surgery.