Ankle valgus after the use of vascularized fibular grafts is a specific complication of the donor site ankle in the growing child. Incidence of this deformity and prevention strategy are documented poorly. The goal of the current study was to evaluate the rate of such deformities and to analyze efficiency of two prevention methods. Charts of 20 children surgically treated for sarcomas of long bones with a mean followup of 4.1 years were studied retrospectively. Ankle valgus was considered if the tibiotalar angle on radiographs with the patient standing was 5 degrees or greater in valgus than that of the opposite ankle, and deformity was considered severe if it required surgical treatment. Prevention was done in some patients with a tibiofibular syndesmotic screw or with reconstruction of the fibula using a tibial autograft. Valgus occurred in nine patients (45%) and was severe in five (25%). Valgus prevention with a syndesmotic screw was efficient and lacking in complications, whereas patients with fibula reconstruction had a high incidence of deformity and relevant complications. The authors recommend using a tibiofibular screw in all patients whose growth plates are open in the lower limbs at the time of surgery.