We compared the results of a radiographic technique for the measurement of instability of the knee with those obtained with a KT-1000 arthrometer. The study was conducted on both knees of sixty patients who had a ruptured anterior-cruciate ligament in one knee, as well as in ten control subjects. The radiographic technique included the examination of a true lateral radiograph, made while the knee was in full extension and the quadriceps was maximally contracted, with a 66.7-newton downward force produced by a 6.8-kilogram weight suspended from the ankle. As demonstrated by both techniques, the maximum difference between the displacements of the right and left knees in the control subjects was 2.5 millimeters and the mean difference between the displacements in the two knees in the patients was 7.5 millimeters. In fourteen of the sixty knees in which the ligament was ruptured, the injury was acute. The forward translation of the medial side in these fourteen knees was compared with that in the forty-six knees in which the injury was chronic. The mean difference in the displacement of the medial side in the right and left knees was 3.5 millimeters in the fourteen patients who had an acute injury and 5.0 millimeters in the forty-six patients who had a chronic injury. Thirteen of the sixty patients had disruption of the posteromedial corner of the injured knee, and the translation of the medial side in these knees was significantly increased compared with that in the intact knees of the same patients.(ABSTRACT TRUNCATED AT 250 WORDS)