The aetiology of functional disorders of the stomatognathic system (TMJ syndrome) still remains a topic of controversy. Two factors, occlusal disharmony and psychologic stress, have been postulated as the main causes of stomatognathic system disturbances. Psychological disorders are considered to be the primary aetiological factors by several authors. Many studies have assessed the personality and emotional characteristics of TMJ patients, using a variety of inventories and clinical interviews. Although the results suggest that a significant percentage of the patients are predisposed to affective disorders, a causative relationship has not been established. Other studies have examined the effects of stress on masticatory muscle activity. The published evidence suggests that patients with TMJ disorders respond with an increased and prolonged masticatory muscle activity when exposed to stressful events. However, other studies have shown that, unlike asymptomatic subjects, patients with stomatognathic disorders failed to show muscular hyperactivity secondary to stressful tasks. Psychological conditions such as depression have also been implicated in an attempt to explain why some patients do not respond to conventional therapy. However, no attempt has been made to approach the matter from an opposite viewpoint, that is to examine whether patients with psychiatric disorders demonstrate a higher prevalence of TMJ disorders. Therefore, the aims, of this study were to investigate: 1.Whether the psychiatric patients develop TMJ disorders in a higher frequency than healthy subjects. 2.Which psychiatric disorder (or disorders) mainly affect the function of the stomatognathic system. 3.Whether a psychological factor is a true cause of TMJ disorders. MATERIALS AND METHOD This study was carried out in the Department of Phsychiatry, NIMTS Hospital of Athens, in collaboration with the Department of the Stomatognathic Physiology, Athens University Dental School. Patients were allocated in two groups: 1.Psychiatric group: This consisted of 58 patients, 24 men and 34 women of age ranging between 30 to 80 years. The group was determined by a psychiatrist and classified with the DSM III criteria of the American Psychiatric Association in the following categories: a.Patients with organic brain syndrome (Patient number: 4) b.Patients with residual schizophrenic disorder (Patient number: 6) c.Patients with paranoid disorder (Patient number: 2) d.Patients with affective disorder: Cyclothymic disorder (Patient number: 4) Dysthymic disorder (Patient number: 19) e.Patients with anxiety disorders: Generalized anxiety disorder (Patient number: 12) Obsessive compulsive disorder (Patient number: 1) f.Patients with adjustment disorders (Patient number: 10) 2.Control group: 24 subjects without any psychiatric disorder, 13 men and 11 women, of age between 40 to 85 years. This group was selected to match the psychiatric group in socio-economic characteristics. Both groups were examined by the principal investigator (dentist) and by the psychiatrist. Dental assessment included a special anamnestic questionnaire and clinical examination of the stomatognathic system. The Anamnestic, Clinical and Occlusal Indices of Helkimo were in this way derived from the findings.