A multicenter study was carried out in 10 Italian Headache Centers to investigate the prevalence of psychosocial stress and psychiatric disorders listed by the IHS classification as the "most likely causative factors" of tension-type headache (TTH). Two hundred and seventeen TTH adult outpatients consecutively recruited underwent a structured psychiatric interview (CIDI-c). The assessment of psychosocial stress events was carried out using an ad hoc questionnaire. The psychiatric disorders that we included in the three psychiatric items of the fourth digit of the IHS classification were depressive disorders for the item depression, anxiety disorders for the item anxiety, and somatoform disorders for the item headache as a delusion or an idea. Diagnoses were made according to DSM-III-R criteria. At least one psychosocial stress event or a psychiatric disorder was detected in 84.8% of the patients. Prevalence of psychiatric comorbidity was 52.5% for anxiety, 36.4% for depression, and 21.7% for headache as a delusion or an idea. Psychosocial stress was found in 29.5% of the patients and did not differ between patients with and without psychiatric comorbidity. Generalized anxiety disorder (83.3%) and dysthymia (45.6%) were the most frequent disorders within their respective psychiatric group. The high prevalence of psychiatric disorders observed in this wide sample of patients emphasizes the need for a systematic investigation of psychiatric comorbidity aimed at a more comprehensive and appropriate clinical management of TTH patients.