Twenty patients, victims of a first luxation of the patella, were the object of a retrospective study. They were all subjected to arthroscopic examination during which, in 7 of the patients, the liquid injected under pressure massively escaped into the subcutaneous layer of the medial side of the knee. The patients were divided into 2 groups according to the integrity of the capsule of their knees. Group I consisted of 7 patients, group II of 13 patients. The treatments, the majority of which were conservative, were quite varied, but the post-traumatic functional recuperation was much slower and more variable for the group I patients. These patients presented more significant tissue lesions which certainly contributed to the functional prognosis of the traumatized joint. Seventy two percent of the patients, equally distributed between the 2 groups, presented trochleo-patellar dysplasia. With the exception of age, different clinical parameters (sex, circumstances of the accident, clinical examination) and radiological criteria (external displacement of the patella, detached bony fragment at the interior border of the patella, value of the patellar and trochlear angles, patella alta) did not permit a distinction between the two groups. Because of our results and those in the existing literature we recommend arthroscopy, which is the only examination capable of establishing a complete picture of the lesions and which, in addition, permits rising of the joint and sometimes allows extraction of free osteo-cartilaginous fragments. The younger patients may then benefit from immediate functional reeducation. For older patients, where the tissue lesions are more important, conservative treatment is associated with a slower and sometimes variable functional recuperation. It remains to be determined, whether surgical treatment, adapted to each case, would permit us to obtain better results more rapidly.