This study evaluated the effects of medial hamstring lengthening on gait in patients with diplegic cerebral palsy. A group of patients who underwent medial hamstring lengthening and distal rectus transfer was compared to a group of patients who underwent intramuscular psoas lengthening as well. Patients who underwent a psoas lengthening showed an increased anterior pelvic tilt and a tendency towards genu recurvatum, features that could indicate postoperative hamstring weakness. These results underline the importance of medial hamstrings for pelvis and knee motion in the sagittal plane during walking. In additional the limitations of the popliteal angle as a clinical measure to evaluate hamstring length in the presence of hip flexor tightness are discussed.