The Q-angle should be the angle generated by a line connecting the anteroinferior iliac spine (AIIS), the bottom of the groove of the femoral condyle, and the tibial tubercle. The Q-angle was measured by computed tomography (CT) in 43 lower limbs with recurrent patellar dislocation and 26 normal controls. The mean Q-angle was 20 degrees in the dislocated group and 13 degrees in controls, a significant difference. No significant difference was found in the Q-angle between the dislocated and control groups using the conventional method. Lower limb rotation had little effect on accuracy of the method.