The outcome of total knee arthroplasty (TKA) with severe pre-operative varus deformity is significantly worse than in well aligned knees. Computer navigated TKA has addressed some of the problems by ensuring accurate post-operative alignment. Our aims were to see if navigation could reproducibly correct a varus deformity to 3 degrees of the mechanical axis and to investigate the relationships between the tourniquet time and severity of the preoperative deformity, BMI and a surgeon's experience. The 172 e.motion floating platform TKA's were implanted using the OrthoPilot Navigation system (B Braun-Aesculap, Tuttlingen, Germany). Pre-operative deformity and post-operative alignment were measured by the software. Tourniquet times were recorded automatically. All knees were corrected to within 3 degrees of the neutral axis (mean 0.48 degrees ). Statistically significant relationships between tourniquet time (TT) and degree of pre-operative varus (p<0.001), total number of previous e.motion TKA's performed (p<0.001), and body mass index (p=0.013) were found. A linear relationship between the variables can be expressed as a statistical formula: TT=49.5+PreOp Varus+0.6(BMI)-0.1(total previous). Tourniquet time is measured in minutes and preoperative varus is measured in degrees from the mechanical axis. Total previous is the total number of navigated TKA's implanted by the surgeon. Tourniquet time is increased with larger pre-operative deformities and high BMI and decreased with surgical experience. The formula may give us a method of predicting the length of a procedure for a particular surgical team and may allow us to plan operating lists more accurately.