Patellar disequilibrium consists of either external malpositioning of the patella (EMP) or external hyperpression (EHP), which are clearly visible on a 30degrees axial radiograph of the knee. Does this disequilibrium play a role in the development of patellar chondropathy? The authors have based their comparisons on a series of 40 subjects more than 60 years of age (average age: 68 years) who were free from pain and arthrosis of the knee. In this group they found 2 cases of EMP (5 percent) and 3 of EHP (7.5 percent). In their series of cases of patellar chondropathy (43 cases) there were, after correction for the cases with a double etiology, 8.5 cases of EMP (20 percent) and 7 of EHP (16 percent). The differences between these figures and those for the control series are statistically significant for the number of cases of EMP (P less than 0.05), but not for the number of cases of EHP, the etiological role of which is probable but for which proof would require a larger series of patients. From this work the following findings emerge: (1) The frequency of latent patellofemoral arthrosis in the aged control subjects with knee problems. 7 cases out of 47, or 12.5 percent. (2) The frequency of patellar chondropathy apparently associated with the two etiological factors: out of 43 patients, 22 suffered from patellar disequilibrium (10 EMP and 12 EHP) but in 13 cases there was another associated possible cause of deterioration: traumatism or stress on the knee associated with work or sport. This methodological difficulty is discussed and resolved empirically. The importance of patellar disequilibrium in the development of degenerative patellofemoral affections derives not from the etiological importance but from the fact that they represent the only factor for which therapy may be given: elective internal rehabilitation or surgery (section of the external wing of the patella or transposition of the anterior tuberosity of the tibia).