The knee was assessed by normal plain radiographs in 20 achondroplasia patients (40 lower extremities), whose age ranged from 10.5 to 23 years. Five patients (seven knees) had permanent patellar dislocation. There was a significant difference between the achondroplasia group and control group (50 knees without skeletal dysplasia) for femoral sulcus angle but not for femorotibial angle, Insall-Salvati index and Q angle. Patellar dislocation for achondroplasia may be caused not only by the abnormalities of soft tissue but also a small groove of patellofemoral joint as a bony malformation.