Posterior laxity of the knee can be assessed clinically, but interpretation of the amount of displacement is highly subjective. Mechanical methods are more efficacious for measuring anterior laxity. Radiologic techniques are available for measurement in a lateral projection, but some variables may interfere with their accuracy. We undertook a trigonometric analysis of the axial view to confirm that it can be used to reliably measure posterior displacement. The ideal radiologic conditions consist of 80 degrees of knee flexion in both knees maintained with a knee support and a 26 degrees x-ray beam incidence with respect to the tibia. Although such accuracy is rarely obtained in routine clinical practice, even with a variability of +/- 10 degrees in the x-ray beam incidence the error factor will be less than 10%. Such a degree of error is in the same range (+/- 2 mm) as noted by investigators using the lateral radiograph to measure anteroposterior displacement. We believe that our study validates the use of a single axial radiograph of both knees to assess the state of the posterior cruciate ligament of an injured knee as compared with a normal knee.