The relationship between impingement of the roof of the intercondylar notch on a reconstructed anterior cruciate ligament, and the subsequent stability and range of extension of the joint, was analyzed in forty-seven knees. The extent of the impingement was determined by analysis of the relationship of the tibial tunnel to the intersection of the line of slope of the intercondylar roof with the plane of the subchondral bone of the articular surface of the tibial plateau. These lines were drawn on a lateral roentgenogram that was made with the knee in maximum extension, two years after the operation. In all four knees in which the entire articular opening of the tibial tunnel was anterior to the slope of the intercondylar roof, there was severe impingement on the graft, and all four grafts failed. In the fourteen knees in which a portion of the articular opening of the tibial tunnel was anterior to the slope of the intercondylar roof, there was moderate impingement on the graft, and four grafts failed (an unacceptable rate of failure). There was no impingement in the knees in which the entire articular opening of the tibial tunnel was posterior to the slope of the intercondylar roof, and these knees were associated with the lowest rate of failure of the grafts (three of twenty-nine). Knees that had an impinged graft and regained a complete range of extension became unstable.