Localization of the centre of the femoral head is vital when using extra-medullary femoral alignment systems in knee arthroplasty. This study investigated the accuracy of a palpable marker placed in the groin for detecting the centre of the femoral head. A table tennis ball, 38 mm in diameter, was taped 2.5 cm perpendicular to the mid-inguinal point in patients having a plain radiograph of the pelvis. The mean horizontal distance between the centre of the table tennis ball and a vertical line passing through the centre of the femoral head was 8.62 mm (range 0-28.6, SD 6.76). In 100% of cases the horizontal distance between the centre of the table tennis ball and a vertical line passing through the centre of the femoral head was less than one diameter of the table tennis ball. Using a theoretical mechanical axis model our clinical method would confer an error equal to or less than 2 degrees from the weight-bearing axis of the knee in up to 98% of cases. In conclusion, our results suggest that the clinical method reported here provides a simple and reliable way of localizing the centre of the femoral head in knee arthroplasty.