For definition of the incidence and natural history of hip flexion contracture, 5,147 serial measurements of the range of hip extension in 966 patients with spina bifida were analyzed. Contractures were generally present in the first few months of life (physiologic flexion posture); this then diminished during the first 27 months in all but those with thoracic lesions. Hence, surgical management is seldom appropriate until after that age. In general, contractures reappear or worsen between the ages of 3 and 6 years, but few patients with sacral lesions develop this deformity. Our findings also demonstrate that hip flexion contracture is not merely due to muscle imbalance, sitting posture, or these factors in combination.