The purpose of this study was to establish the incidence of the anatomic structure, the popliteal bursa, in patients undergoing arthroscopy and to determine the relationship to associated clinical and pathological factors. Diagnostic arthroscopy was used to identify the presence of the popliteal bursa in 187 consecutive patients (195 knees). Thirty-seven percent of knees had a popliteal bursa identified by the communication with the posterior medial compartment. The cause of the popliteal bursa was not established by this study. This study refuted the causes proposed by others. The existing erroneous conclusions concerning the cause of the popliteal bursa were probably attributable to the failure to recognize the common continuity of the bursa and the knee joint. When the popliteal bursa is present (37%), it becomes symptomatic by responding to the intraarticular disease because of its continuity with the knee joint.