Background. Computer-assisted navigation systems for total knee arthroplasty (TKA) were introduced to improve implantation accuracy and to optimize ligament balancing. Several comparative studies in the literature confirmed an effect on the component position and other studies could not confirm these results. For ligament balancing most studies found no significant influence on the clinical outcome using a navigation system for TKA. In the literature there were no reports of mid-term results after navigated TKA. With our study we wanted to show if the use of a navigation system for TKA will have an influence on the component's position and on the clinical results at 5-year follow up. Methods. We enrolled 200 patients in a prospective randomized study with a minimum follow up of 5 years. 100 TKA were operated on without using a navigation system (Group A) and 100 surgeries (Group B) were done with computer assistance. Radiological investigation by standard radiographs including a long-leg X-ray was performed with a follow up rate in Group A of 86.2% versus 80.2% in Group B. We measured the mechanical axis of the leg, lateral distal femoral angel (LDFA), medial proximal tibial angle (MPTA), tibial slope and the alpha-angle of the patella. Clinical investigation was performed with a follow up rate in Group A of 85.7% versus 79.8% in Group B including the parameters for the range of motion (ROM), ligament balancing, anterior drawer test, feeling of instability, anterior knee pain, effusion, WOMAC Score, Insall Score and HSS Knee Score. Results. In both groups there was no aseptic loosening or difference in TKA survival rate (Group A 95.4% versus Group B 98.85% 5-year survival rate, p = 0.368). With the navigation technique the mechanical axis of the limb in the frontal plane was improved (p = 0.015; Group B: 1.67 ° ± 1.6° versus 2,44 ° ± 2.2 ° in Group A). 90% of the Group B and 81% of the Group A were within 3 ° varus/valgus deformity of the mechanical axis of the limb (p = 0.157). The accuracy of tibial slope was higher in the Group B (p = 0.001). More patients of the Group B (95% versus 79%) were within a deviation of 3 °, −7 ° tibial slope (p = 0.007). The mean deviation of 90 ° LDFA was higher (p = 0.034) in the Group A (1.89 ° versus 1.36 ° in Group B). Mean deviation of 90 ° MPTA, mean MPTA, mean LDFA and patella alpha angle were similar in both groups (p 0.253). There was no difference in ROM, ligament balancing, anterior drawer test, anterior knee pain or feeling of instability (p 0.058). Insall Knee Score total (181 Group A/191 Group B) and HSS Knee Score total (91 Group A/93 Group B) was higher with the navigated procedure in Group B (p 0.026). WOMAC total and HSS grades were similar in both groups (p 0.070). Conclusions. The accuracy of the mechanical axis of the limb and the tibial slope was higher with the navigated procedure. TKA survival rate and clinical outcomes were similar in both groups at 5-year follow up