The suprapatellar plica or incomplete suprapatellar septum of the knee is a recognized cause of knee pain in some patients and appears to exist in others without causing any symptoms. To date no clear method of demonstrating the pathomechanics of the suprapatellar plica by arthroscopic means has been described. This article describes a new technique of visualizing this structure arthroscopically and demonstrating the mechanism of its morbidity. By the use of a proximal viewpoint via the lateral suprapatellar portal, we have shown that the suprapatellar plica impinges on the medial femoral condyle and is trapped between the quadriceps mechanism and the femoral trochlea when the knee is flexed beyond 70 degrees. We conclude that our arthroscopic method may show the mechanism of production of symptoms of the pathological suprapatellar plica.