Trochlear dysplasia is a well-known cause for recurrent patellar instability. Besides clinical findings, the treatment is based on radiological diagnostic tools. In adults the characteristics of trochlear dysplasia are determined by magnetic resonance imaging (MRI) scans as well as on true lateral radiographs. For children there are no established criteria for the radiological diagnosis of trochlear dysplasia. This study was designed to evaluate if typical radiological findings of trochlear dysplasia on lateral radiographs in adults are also found in children and adolescents with open growth plates. We analyzed true lateral radiographs of children and adolescents with MRI-detected trochlear dysplasia. On lateral radiographs three factors were relevant: crossing sign (deepest part of the trochlea crosses the most anterior point of the lateral condyle), a supratrochlear spur or bump (bulge of the proximal trochlea), and a double contour (due to a hypoplastic medial condyle). In all patients with trochlear dysplasia at least one typical radiological finding usually found in adults could be documented. Only true lateral radiographs allow the diagnosis of trochlear dysplasia in children and adolescents with open physis to be made. MRI scans on a regular basis are, therefore, not mandatory although they are the diagnostic golden standard for the diagnosis of trochlear dysplasia.