Background and purpose - Manipulation and cast immobilization is the primary management for diaphyseal forearm fractures in children, and re-displacement is the most common complication. We wanted (1) to analyze the incidence of re-displacement in a group of children treated with close reduction and casting; (2) to determine predictive factors such as demographics, mechanism of injury, affected bone, fracture pattern, degree of initial displacement and angulation, and reduction accuracy; and (3) to determine the prognostic effect of previously defined radiographic indices. Patients and methods - We prospectively studied 269 consecutive children with closed and complete middle-third diaphyseal fractures treated with close reduction and casting from October 2014 to April 2015. Factors analyzed included demographics, initial fracture features, having a non-anatomical reduction, and the radiographic indices of cast quality. Results - There were 189 fractures of both bones (70%) and 80 solitary fractures (30%). The overall re-displacement rate was 11%. According to multivariable analysis, independent predictors of re-displacement were initial angulation >10° (RR =5) and failure to achieve an anatomical reduction (RR =2). Statistically significant radiographic indices regarding increased rate of re-displacement included cast index ≥0.7 (RR =5), Canterbury index ≥1.1 (RR =3), and 3-point index ≥0.8 (RR =6). Interpretation - Our results suggested that fractures with a higher degree of initial angulation and non-anatomical reduction more often result in re-displacement. Moreover, the casting quality examined with the radiographic indices played an important role in the success of a non-operative management.