The material of this investigation consisted of 104 patients (37 males and 67 females), treated for dislocation of the patella at the Department of Orthopedics in Lund during 1975-1977. The patients were examined clinically and radiographically in order to identify the etiologic importance of trauma, generalized joint laxity, and an anatomically abnormal patellar articulation. All patients were examined for generalized laxity, other orthopedic conditions, relatives with patellar dislocation or congenital dislocation of the hip, dislocation frequency, bilateral patellar dislocation, age and the nature of the trauma at initial dislocation, incidence and type of articular fractures associated with patellar dislocation, and three radiographic parametres of anatomically abnormal patellar articulations. Generalized joint laxity was observed in two thirds of the patients, frequent dislocations in one half and bilateral dislocations in one third. In half of the patients the initiating trauma was insignificant, and one third had avulsion or osteochondral fractures. The majority had definite anatomic abnormalities but patella alta was observed in only one fourth of the patients. The material was subjected to an analysis of the covariation of etiologic factors operating in patellar dislocation. On this basis the material was classified in four grades of increasing patellar instability, from Grade I with neither laxity, nor increased Insall index, to Grade IV with both these factors. With increasing patellar instability the frequency of recurrent and bilateral dislocations increased whereas the degree of trauma and incidence of fractures decreased. The type and degree of anatomic abnormalities were correlated to the different grades of patellar instability. The classification of patellar instability proposed here should permit more precise evaluation of treatment of patellar instability, and, eventually, provide a basis for the choice of therapy in patellar dislocation.