Although the intercondylar notch becomes narrowed with a failed repair of the ACL or with chronic instability of the knee, narrowed intercondylar notches have frequently been observed during arthrotomy for an acute ACL tear in previously uninjured knees. In order to determine whether the structure of the intercondylar notch predisposes a knee to anterior cruciate tears, a computer graphics study was designed. Notch view roentgenograms of 50 patients with an acute ACL injury and 50 "normal" patients without history of significant knee injury were compared. An interactive graphics design station was used to obtain the area of the anterior opening (ANT) and posterior arch (POST) of the intercondylar notch. A "total" (TOTAL) area was obtained by tracing the distal 7.5 cm of the femur. Only the ratios of areas obtained from each roentgenogram were used for comparison because of varying femur sizes among patients and possible slightly varying techniques of taking the notch view roentgenogram itself. The following ratios were developed: ANT/POST, POST/TOTAL, and (POST-ANT)/TOTAL. The difference between the means of the ratio POST/TOTAL for knees with acute ACL tears and those for normal knees was statistically significant (P less than 0.005). This study suggests that a narrowed posterior arch of the intercondylar notch may predispose a knee to ACL tear.