The aim of this study was to assess the angular components of the affected foot associated with valgus deformity of the unaffected foot and to redefine the actual leg-length inequality in unilateral cerebral palsy. We retrospectively reviewed the medical records and radiologic images of 76 patients with unilateral cerebral palsy. Weight-bearing plain radiography of both feet of each subject was obtained. Angular measurements focused on the collapse of the longitudinal arch, hind foot valgus and forefoot abduction. Patients were divided into two groups: with and without valgus deformity of the unaffected side. Leg-length discrepancy and pelvic obliquity angle were measured Among 76 patients, 40 (52%) had valgus deformities of the unaffected side. Independent t-test revealed no significant differences in age, affected side, type of deformity on the affected side, or application of bilateral biomechanical foot orthosis between patients with or without valgus deformity of the unaffected side. Patients with valgus deformity had significantly increased voluntary ankle dorsiflexion greater than neutral on the affected side, leg-length discrepancy and lateral talocalcaneal angle (P < 0.05). Laterally measured foot angles of both feet were significantly correlated. The optimal cut-off points for predicting valgus deformity were leg-length discrepancy >10 mm or affected limb/unaffected limb-length index <0.98. Leg-length discrepancy and lateral talocalcaneal angle of the affected foot were significantly increased in patients with valgus deformity of the unaffected side. The optimal cut-off point for predicting valgus deformity of the unaffected foot would be useful in clinical practice.