BACKGROUND: Irritable bowel syndrome (IBS) is a prevalent functional gastrointestinal disorder. Previous studies have shown that the burden of illness of IBS is high. It has been suggested that the quality of life and medical costs of IBS patients are associated with the severity of pain. This study aimed to quantify the burden of illness of IBS in relation to the severity of IBS symptoms. METHODS: Patients identified at community pharmacies as mebeverine users were administered a questionnaire regarding 1) the Rome II criteria for IBS, 2) predominant type of stool during symptomatic episodes, 3) severity of symptoms (abdominal pain and discomfort), 4) generic (SF-36) and disease-specific quality of life (IBS-QOL), 5) current health status (utilities, EQ-5D and SF6D), and 6) loss of productivity. Associations between severity of symptoms and burden of illness (including SF-36, IBS-QOL, EQ-5D, SF6D, direct medical cost and loss of productivity) were assessed. RESULTS: For 168 patients, who met the Rome II criteria for IBS, information on severity of symptoms was available. The majority of patients (98, 58%) were categorized as having severe symptoms of IBS. 47 (28%) patients were suffering from moderate symptoms and 23 (14%) patients had mild symptoms or were asymptomatic. All components of SF-36 scored lower as the symptom severity increased. The IBS-QOL score was lower (71.4, 95%CI: 67.5-75.3) for patients with severe symptoms than for patients with moderate IBS and mild/asymptomatic IBS (81.8, 95%CI: 78.6-85.0 vs. 81.3, 95%CI: 75.5-87.1). The EQ-5D score was also lower (58.3, CI95% 59.5-65.3) for patients with severe symptoms compared to the two other groups (68.8 and 66.3). CONCLUSIONS: This study clearly indicates that the burden of illness of patients with IBS increases with increasing severity of symptoms.