Background and objective: Irritable bowel syndrome (IBS) and somatization disorder (SD) are defined by nonobjective symptoms that overlap considerably. Psychiatric symptoms associated with IBS may originate from SD in IBS patients. Previous studies of IBS have not considered SD separately from IBS. Methods: This study explored psychiatric symptoms and illness behavior in IBS in relation to SD. A total of 50 outpatients with IBS or ulcerative colitis (UC) were evaluated with the Diagnostic Interview Schedule and Illness Behavior Questionnaire. Results: Definite or probable SD was diagnosed in no UC patients and in 42% of IBS patients (confirmed in 25% and lacking one symptom in another 17%). IBS patients with probable or definite SD, but not those without SD, reported more psychiatric symptoms and abnormal illness behaviors than did UC patients. SD accounted for the association of psychiatric symptoms with IBS. Conclusions: In this university-based office setting, the association of psychiatric features with IBS appears heterogeneous predicated on whether SD is present. Future studies of functional bowel diseases should distinguish between patients with and without SD to clarify its relationship to these disorders. Clinicians should consider whether patients with functional disorders have SD, a diagnosis that indicates specific clinical management strategies.