Primary malignant bone tumor often requires a surgical treatment to remove the tumor and sometimes restore the anatomy using a frozen allograft. During the removal, there is a need for a highest possible accuracy to obtain a wide safe margin from the bone tumour. In case of reconstruction using a bone allograft, an intimate and precise contact at each host-graft junction must be obtained (Enneking 2001). The conventional freehand technique does not guarantee a wide safe margin nor a satisfying reconstruction (Cartiaux 2008). The emergence of navigation systems has procured a significant improvement in accuracy (Cartiaux 2010). However, their use implies some constraints that overcome their benefits, specifically for long bones. Patient-specific cutting guides become now available for a clinical use and drastically simplify the intra-operative set-up. We present the use of pre-operative assistances to produce patient-specific cutting guides for tumor resection and allograft adjustment. We also report their use in the operative room. We have developed technical tools to assist the surgeon during both pre-operative planning and surgery. First, the tumor extension is delineated on MRI images. These MRI images are then merged with Computed Tomography scans of the patient. The tumor and the CTscan are loaded in custom software that enables the surgeon to define target (desired) cutting planes around the tumor (Paul 2009) including a user-defined safe margin. Finally, cutting guides are designed on the virtual model of the patient as a mould of the bone surface surrounding the tumor, materialising the desired cutting planes. When required, a massive bone allograft is selected by comparing shapes of the considered patient's bone and available allografts. The resection planes are transferred onto the selected allograft and a second guide is designed for the allograft cutting. The virtually-designed cutting guides are then manufactured by a rapid prototyping machine using biocompatible material. This procedure has been used to excise a local recurrence of a tibial sarcoma and reconstruct the anatomy using a frozen tibial allograft. The pre-operative planning using virtual models of the patient's bone, tumor and the available allografts enabled the surgeon to localise the tumor, define the desired cutting planes and select the optimal allograft. Patient- and allograft-specific guides have been designed and manufactured. A stable and accurate positioning of guide onto the patient's tibia was made easier thanks to the plate formerly put in place during the previous surgery. An accurate positioning of the allograft cutting guide has been obtained thanks to its design. The obtained reconstruction was optimal with a adjusted allograft that was perfectly fitting the bone defect. The leg alignment was also optimally restored. Computer assistances for tumor surgery are progressively appearing. We have presented at CAOS 2010 an optical navigation system for tumor resection in the pelvis that was promising. However, such a tool is not well adapted for long bones. We have used patient-specific guides on a clinical case to assess the feasibility of the technique and check its accuracy in the resection and reconstruction. The surgeon has benefited from the 3D planning to define his strategy. He had the opportunity to select the optimal transplant for his patient and plan the same cuttings for the allograft and the patient. During the surgery, guide positioning was straightforward and accurate. The bone cuttings were very easy to perform. The use of custom guides decreases the operating time when compared to the conventional procedure since there is no need for measurements between cutting trajectories and anatomical landmarks. Furthermore, the same cutting planes were performed around the tumor and onto the allograft to obtain a transplant that optimally fills the defect. We recommend the use of such an intra-operative assistance for tumor surgery