The purpose of this study was to assess under- and overcorrection following femoral derotation osteotomy (FDO) in spastic diplegic children with functionally compromising internally rotated gait, especially with respect to asymmetry. A total of 30 children with spastic diplegia and internally rotated gait underwent multilevel surgery including FDO and were assessed pre- and 1 year postoperatively by clinical examination and instrumented gait analysis. The amount of derotation was determined intra-operatively based on the neutral midpoint between passive internal and external hip rotation and was controlled with K-wires. Sixteen patients showed an asymmetry in mean hip rotation in stance of more than 10 degrees preoperatively. Accordingly, all legs were classified as the more or the less involved side. Improvement parameters were established for the evaluation of over- and undercorrection. For the clinical midpoint, no significant difference in change and improvement was found between the sides. The mean hip rotation in stance improved significantly in the more involved side with few incidences of mal-correction (13%). The less involved side, however, got worse due to a high occurrence of over- and undercorrections (59%). Hence the clinical midpoint as indication criterion does not give sufficient information about the functional alignment of the distal femur during gait, especially in legs with mild involvement. The study underlines the importance of gait analysis for indication in addition to the clinical midpoint. Taking into account the precision of gait analysis data and derotation amount, we suggest FDO to be carried out if the mean internal hip rotation in stance exceeds 15 degrees .