Because previous methods of making roentgenograms of the knee in the axial projection were found to be unsatisfactory, a simple, accurate, and reproducible technique was devised. With the patient supine, the knees are flexed 45 degrees over the end of the table and the legs are supported. The central x-ray beam is inclined downward 30 degrees from the horizontal to strike the film cassette (placed distal to the patellae) at a right angle. The congruence angle, which measures the relationship of the patellar articular ridge to the intercondylar sulcus, was created to measure patellofemoral joint congruence. More than 16 degrees of lateral subluxation of the patella was found to be abnormal at the ninety-fifth percentile in 100 normal subjects.