Aims: To study the utility of a computer assisted orthopaedic surgery (CAOS) wireless system (navigator) in Total Knee Arthroplasty (TKA). Methods: Randomised prospective study. A sample of 40 TKA patients was randomised in two groups: CAOS was used in 20 of them. In the other group standard technique with manual alignment was performed. Femoral angle (formed between the femoral mechanical axis and the femoral component), tibial angle (formed between the tibial mechanical axis and the tibial platform) and femorotibial angle (formed between femoral and tibial mechanical axes) were measured from Computed Tomography Surviews taken in the immediate postoperative period. Results: In the standard group (without navigator) the femoral angle mean was 91.7° (ranged 90 to 94°). Tibial angle mean was 90.2° (87°–95°) and femorotibial angle mean was 175.9° (172°–180°) showing a slight prevalence of varus deviation of the extremity mechanical axis. In the group with navigator the femoral angle mean was 90.2∞ (87–93°), tibial angle mean 89.6°(85°–93°) and femorotibial angle mean 179.2° (177°–182°). There were statistically significant differences between groups for the femoral angle (p=0.001), and the femorotibial angle (p < 0.001). An ideal femorotibial angle (180±3°) was achieved for all the patients of the CAOS group but only 9 patients of the standard technique group reached this objective (p< 0.001). Conclusions: The use of CAOS for TKA favors the implant placement in a position nearer to the ideal mechanical axis.