The aim of this study was to retrospectively determine the risk factors for delayed union in 117 consecutive pediatric both-bone forearm fractures treated with internal fixation. Eight patients (7%, 8/117) had delayed unions, all were boys treated with intramedullary fixation for a fracture in the middle-third of the bone; and in seven patients, the ulna was the site of the delayed union. Older age, double-bone fixation, increased initial fracture displacement, and opening a closed ulna fracture were associated with longer time to union (P<0.05). Identification of risk factors will aid in the selection and duration of internal fixation and duration of immobilization.