Introduction. Displaced distal radius fractures in children have been treated in above elbow plaster casts since the last century. Cast index has been calculated previously, which is a measure of the sagittal cast width divided by the coronal cast width measurement at the fracture site. This indicates how well the cast was moulded to the contours of the forearm. We retrospectively analysed the cast index in post manipulation radiographs to evaluate its relevance in redisplacement or reangulation of distal forearm fractures. Study Design. Consecutive radiographic analysis. Materials and methods. 156 consecutive paediatric patients (114M : 42F), with a mean age of 9.8 years (range 2-15 years), presenting with forearm fractures were studied. All patients were manipulated in OR and a moulded above elbow cast was applied. The cast index was measured on immediate post manipulation radiographs. Results. Displacement of the fracture within the original plaster cast occurred in 30 patients (19%), 22/114 males; 8/42 females. The cast index in the 30 patients requiring a second procedure (mean 0.92, SD=0.08) was significantly more than the cast index (mean 0.77, SD=0.07) in the others (p< .001). Discussion. A high cast index in post manipulation radiographs indicates increased risk of re-displacement of the fracture and these patients should be kept under close review. Conclusion. Cast index is a valuable tool to assess the quality of moulding of the cast following closed manipulation of distal radius fractures in children. The maximum acceptable cast index should be 0.82