Radiologic criteria are commonly used to diagnose femoral trochlear dysplasia causing patellofemoral (sub)luxations. A dysplastic trochlea can be identified on conventional radiographs when the line of the trochlear groove crosses the anterior border of one or both condyles. The aim of this study was to establish the influence of rotation on the precision of these radiographic criteria as classified on a true lateral conventional radiograph. In a radiological in vitro study using two different distal femurs we found that rotation deviations of the distal femur can simulate femoral trochlear dysplasia in a normal knee. In case of trochlear dysplasia, rotational deviations can simulate a normal trochlear shape with crossing of the trochlear groove with the anterior border of the femoral condyle. Even a rotation deviation of 5 degrees can cause a false-positive or false-negative diagnosis. We recommend the use of fluoroscopy to obtain a true lateral view (with both condyles overlapping on the posterior side) in order to correctly classify trochlear dysplasia and would strongly urge that no surgery should be performed on the basis of rotated conventional radiographs.