A retrospective analysis of 540 operated cases (740 joints) of internal derangements of the temporomandibular joint was carried out. Observations of this patient population provided the basis for describing pathological variations of internal derangements. Radiologic studies, including tomograms and arthrograms or magnetic resonance scans, and surgical/pathological findings were correlated with clinical data in each case. It was found that clinical manifestations varied in a characteristic way and were directly related to the degree of pathological change and time course. Various stages of internal derangements were identified. Pathophysiological mechanisms responsible for the observed changes, as well as clinical signs and symptoms and causal factors, were discussed. Internal derangements are organic lesions that appear to be progressive and are probably of traumatic origin. The view is given that internal derangements represent the basic pathological entity responsible for clinical manifestations of what has been known as the temporomandibular joint pain-dysfunction syndrome or similarly described conditions. Effective clinical management takes on new importance because progression to advanced degenerative states may occur.