The methodological detail about determining the femoral axis on lateral fluoroscopic images of the knee is lacking. This paper reports on the use of the posterior border of the lower femoral diaphysis to represent the axis. This method is accurate, simple, reliable and can be used by researchers studying knee joint kinematics using dynamic video fluoroscopy. It is also useful in the outpatient clinic to assess the flexion/extension position of femoral components after knee arthroplasty.