The patella is one of the most important components of the knee extension mechanism, and position of patella relates to some diseases of the knee, especially recurrent dislocation of the patella and Osgood-Schlatter s diseases. We checked the patella to patella tendon ratio in normal persons as well as in cases with recurrent dislocation of patella, chondromalacia of patella, degenerative arthritis of the knee and Osgood Schlatter s disease by means of Insall and Salvati method. The following results were obtained: 1) The ratio was 1.048 in normal persons. 2) The ratio was 0. 845 in recurrent dislocations. 3) The ratio was the same as normal in cases of the acute stage of Osgood-Schlatter's disease but chenged to 1. 236 in the late healing stage after 18 years of age. 4) The ratio in chondromalacia of the patella and degenerative arthritis of the knee was not significantly different from the normal.