This study investigated the frequency and distribution of clinical signs of temporomandibular joint (TMJ) internal derangement in an adult non-TMJ patient population. Four hundred three persons who participated in an epidemiologic investigation were examined for clinical signs of TMJ internal derangement by four examiners who followed a standardized form. Clinical signs of internal derangement were found in 76 persons (19%). Twenty-nine persons (7%) had reciprocal clicking and 47 (12%) had a history of clicking replaced by limitation of mouth opening with deviation to the affected side. Reciprocal clicking was associated with TMJ pain during mouth opening and with limitation of jaw movement. A history of clicking replaced by limitation of mouth opening with deviation to the affected side was associated with pain during mouth opening, limitation of opening, and palpatory tenderness of the TMJ. The study indicates that clinical signs of TMJ internal derangement are present in nearly one fifth of non-TMJ patients. Those with clinical signs of internal derangement frequently also have subjective symptoms but they have not sought treatment for these symptoms.