Psychological difficulties in patients with irritable bowel syndrome (IBS) are strongly related to symptom severity and patient status. This has important implications for clinical practice, and the design and conduct of clinical trials. Psychosocial factors (personality, psychiatric diagnosis illness behavior, life stress, psychological distress) distinguish patients with IBS from patients with no IBS. Psychosocial difficulties (e.g., history of physical or sexual abuse, maladaptive coping, or "catastrophizing") predict poorer health outcome (greater pain scores, psychologic distress and poorer daily function, more days spent in bed, and more frequent physician visits and surgeries). When using the standardized Functional Bowel Disorder Severity Index, patients classified as severe are distinguished from moderates by several psychosocial difficulties and health-care use variables. In addition, whereas patients with severe illness report more pain, there is no difference from patients with moderate illness in terms of visceral sensation threshold. Given these data, it is important to consider psychosocial factors as predictive of symptom severity and clinical outcome, and this should be considered in clinical care and the design of clinical trials.