The aim of this study was to clarify the effects of general anesthesia (GA) on joint range of motion (ROM) in children with spastic cerebral palsy (SCP). Eighty-four SCP cases (mean age 8.4 years) admitted for first corrective surgery were retrospectively reviewed. Lower limb ROM were measured 1 day before operation and immediately after GA. Contracture of hip, knee, and ankle joints decreased significantly after GA, with + 11.1° (39.5%) for the hip abduction angle, -3.7° (18.0%) for the Thomas test, -15.0° (19.1%) for the popliteal angle, + 6.6° (39.8%) and 7.0° (109%) for ankle dorsiflexion with knee flexion and extension, respectively (all P < 0.001). These changes were correlated positively to pre-GA contracture and body weight, negatively to age, but independent of preoperative functional level, geographic classification of SCP, or modified Ashworth scale. On the basis of these findings, routine post-GA reassessments of joint ROM before corrective surgeries were recommended for pediatric SCP cases.