This study describes lesions that occur in the stifle joints of dogs with patellar luxation. These lesions are associated with the animal's age, body weight and degree of luxation. It also reports on the rate of re-dislocation. The patellar lesions found include articular cartilage erosion, subchondral bone exposure, a flattened or concave patellar surface and enthesophytes. Extrapatellar lesions included synovitis, osteophytes, blunting of the trochlear groove, an absent trochlea, erosion of the condylar margins, capsule thickening, a long digital extensor tendon injury, "joint mice," flap formation, cranial cruciate ligament rupture and meniscal prolapse. Such lesions were frequently found in animals with grade II or III luxation that were aged 24 months or older; they were more severe in dogs weighing more than 15 kg. Patellar luxation causes changes that favour articular degeneration and should be treated surgically. Conservative treatment relieves pain but does not address tissue alterations.