Objectives Patients' ideas about the nature, cause, and treatment of their illnesses are part of the complex process of coping with illness. To date, limited research on subjective theories of illness in patients with irritable bowel syndrome (IBS) has been performed. The aim of the study was to investigate patients' subjective theories of illness and how these are related to clinical and psychological outcome criteria, in particular IBS symptom severity and quality of life. Methods Eighty-eight patients with IBS, as defined by Rome III criteria, were administered a battery of questionnaires to collect the following data: sociodemographic variables, subjective theories of illness (Subjektive Krankheitstheorien, Cause Questionnaire), anxiety (Hospital Anxiety and Depression Scale), depression (Beck Depression Inventory), quality of life (SF-12), and IBS symptoms (Questionnaire for Gastrointestinal Symptoms). Results Almost all patients reported theories of illness reflecting their subjective causal assumptions. The most frequently mentioned causal factors were physical illness, intrapsychic factors, and stress. Patients with mainly somatic attributions had higher IBS symptoms scores (P<.05) and reduced physical quality of life. Intrapsychic attributions were associated with reduced mental quality of life and enhanced physical quality of life (P<.01). All correlations were independent of gender, age, and irritable bowel subgroups. Conclusions Subjective theories of illness can have significant implications for IBS symptom severity, as well as for physical and mental quality of life.