Ninety-two patients with displaced Colles' fractures were followed prospectively after manipulation under regional anaesthesia. Radiographs were taken before and after manipulation and after 3 months when a functional assessment was also made. A correlation was sought between outcome and radiological measurements. Final radial shortening of 3mm or more was associated with a significantly worse functional outcome after 3 months (P < 0.001). Where the initial radial shortening was 3 mm or more, the probability of malunion was 65% whereas with less than 3mm, the probability of malunion was 28% (P < 0.001). With 5 mm or more of radial shortening at presentation, the probability of malunion was 73% (P < 0.01). The decision to manipulate remains a matter of judgement but a high failure rate renders simple manipulation and plaster cast fixation a poor treatment option in fractures with 5mm or more of radial shortening at presentation.