Objective: In this study we measured the position of the anatomical axis of the distal femur relative to the articular surface of the femoral condyle in lateral radiography, and compared the position of the anatomical axis to the position of the femoral stem for total knee arthroplasty. Methods: We studied 81 knees in 81 rheumatoid arthritis patients. The radiological grade of each knee joint was assessed by the Larsen method, and only knees for which Larsen grade was •...III were included. The anatomical axis was defined as a line connecting the midpoints of femoral A/P diameter at 6 cm and 12 cm proximal from the distal end of the femur. The distances between that anatomical axis and the lines tangent to the anterior cortex and to the posterior condyle parallel to the anatomical axis were defined as A and B, respectively. The distances between the midline of the stem and the lines tangential to the inferior border of the anterior flange and to the posterior flange parallel to the midline of the stem were defined as a and b in 5 kinds of prostheses. The ratios A/B and a/b were calculated. Next, the proper size of each kind of prosthesis for each patient was determined by laying the template on the lateral radiograph. The distance between the midline of the stem and the anatomical axis was calculated as a minus A.