PURPOSE: To evaluate the influence of thigh weight in different hip flexion positions on the knee flexion gap in total knee arthroplasty (TKA). MATERIALS AND METHODS: We evaluated 20 patients (25 knees) with osteoarthritis of the knee that underwent TKA using a navigation system from May 2010 to April 2011. After posterior cruciate ligament sacrificing, complete soft tissue balancing, and fixation of all components with cement, the flexion gaps were measured with the patello-femoral joint reduced. Medial and lateral flexion gaps were measured separately in both the 90degrees-90degrees and 45degrees-90degrees flexion positions of the hip-knee joints. RESULTS: The medial and lateral flexion gaps in the 45degrees-90degrees flexion position of the hip-knee joints were 13.02+/-2.17 mm and 13.12+/-2.21 mm, respectively. The medial and lateral flexion gaps in the 90degrees-90degrees flexion position were 12.92+/-2.03 mm and 13.08+/-2.29 mm, respectively. The flexion gaps showed no significant (p>0.05) differences between the two different hip flexion positions. CONCLUSIONS: Flexion gaps in TKA were not influenced by hip flexion positions (45degrees or 90degrees of flexion). Therefore, soft tissue balancing and polyethylene thickness should not be affected by hip flexion positions during TKA.