An objective method of evaluating surgical margins and the extent of limb-preserving surgery is described in a study of 10 malignant bone tumours. The resected specimen was imaged by computerised tomography and magnetic resonance, and the appearances compared with the preoperative images of the tumour. CT was better than MR for detecting bony changes, but MR was more satisfactory for showing the extraosseous and intramedullary outline of the tumour. These two methods should therefore be combined. Intralesional excision is an absolute indication; marginal excision a relative indication for reoperation.