Previous research has established that patients with irritable bowel syndrome (IBS) frequently have comorbid psychiatric disorders. This study sought to establish if the timing of the onset of psychiatric disorders indicated that IBS was more likely to be caused by or more likely to cause psychological difficulties. Participants were 188 treatment-seeking IBS patients who were assessed for psychiatric diagnoses using a semistructured clinical interview. Timing of the onset of any lifetime psychiatric disorders was noted in an attempt to determine if psychiatric disorders were more likely to precede or follow the onset of IBS symptoms. Those participants that met criteria for an Axis I disorder sometime during their life were significantly more likely to develop an Axis I disorder before the onset of IBS symptoms. Anxiety disorders were the most likely disorder to develop before IBS. These results support the theory that psychiatric symptoms, especially anxiety, play a role in the development of IBS.