Malalignment in total knee replacement (TKR) is frequently associated with earlier failure and poor functional results. The authors compare the radiological results achieved in three consecutive series of TKRs using a computer-based alignment system (38 cases), a totally intramedullary alignment system (40 cases) and a totally extramedullary alignment system (37 cases). The frontal-femoral-component angle (FFC), the frontal-tibial-component angle (FTC), the hip-knee-ankle angle (HKA) and the sagittal orientation of the tibial component (slope) were evaluated 12 months after operation. The results did not show any statistically significant differences between the mean values of FFC, FTC, HKA angles and tibial slope among the three groups. However in the extramedullary alignment group there was a statistically higher percentage of TKRs with abnormal FFC and HKA angles. Furthermore all the implants in the computer aligned group were aligned within 4 degrees both of an ideal HKA and tibial slope.