The purpose of this study was to investigate the effectiveness of lateral column lengthening in the treatment of flexible, planovalgus foot deformity of ambulatory children with spastic cerebral palsy (CP). Fifteen ambulatory children (23 feet) with spastic CP and flexible planovalgus foot deformities received lateral column lengthenings through the calcaneus or calcaneocuboid joint. At an average of 4.1 years of follow-up (minimum, 2.3 years), 17 (74%) feet had good, two (9%) had fair, and four (17%) had poor outcomes. Poor outcomes resulted from recurrence of deformity. Improvements in preoperative, postoperative, and follow-up radiographic measurements were found. However, these improvements are not related to the clinical outcomes. Calcaneal lengthening is a successful treatment for flexible planovalgus foot deformity in ambulatory children with spastic CP. The high failure rate is a significant limitation to the procedure. Further investigation of the objective measurement of planovalgus foot deformity is needed.