To compare the results for patients treated with intercalary endoprosthetic replacement (EPR) or intercalary allograft reconstruction for diaphyseal tumours of the femur in terms of: (1) reconstruction failure rates; (2) cause of failure; (3) risk of amputation of the limb; and (4) functional result. Patients with bone sarcomas of the femoral diaphysis, treated with en bloc resection and reconstructed with an intercalary EPR or allograft, were reviewed. A total of 107 patients were included in the study (36 EPR and 71 intercalary allograft reconstruction). No differences were found between the two groups in terms of follow-up, age, gender and the use of adjuvant chemotherapy. The probability of failure for intercalary EPR was 36% at 5 years and 22% for allograft at 5 years (p = 0.26). Mechanical failures were the most prevalent in both types of reconstruction. Aseptic loosening and implant fracture are the main cause in the EPR group. For intercalary allograft reconstructions, fracture followed by nonunion was the most common complication. Ten-year risk of amputation after failure for both reconstructions was 3%. There were no differences between the groups in terms of the mean Musculoskeletal Tumor Society score (27.4, range 16-30 vs. 27.6, range 17-30). We have demonstrated similar failure rates for both reconstructions. In both techniques, mechanical failure was the most common complication with a low rate of limb amputation and good functional results. Level III, therapeutic study.