Objective. To study the prevalence of irritable bowel syndrome (IBS) and its comorbidity in a Norwegian adult population.Material and methods. In 2001, 11,078 inhabitants (aged 30-75 years) in Oppland County were invited to take part in a public health survey. A total of 4622 subjects (42%) completed the questionnaires on symptoms of IBS (Rome II criteria), comorbidity, health-care visits and medications. The impact of comorbidity on global health, working disability and use of health-care resources in subjects with IBS was explored by stepwise logistic regression.Results. The population prevalence of IBS was 388/4622 (8.4% (95% CI: 7.6-9.4%)) with a female predominance and an age-dependent decrease. The proportion who had consulted for IBS ranged from 51% among 30-year-olds to 79% in 75-year-olds (p=0.05). IBS was associated with musculoskeletal complaints (OR = 2.4-3.4 for six different items), fibromyalgia (OR = 3.6 [2.7-4.8]), mood disorder (OR = 3.3 (2.6-4.3)), reduced global health (OR = 2.6 (2.1-3.2)), working disability (OR = 1.6 (1.2-2.1)), more frequent health-care visits and use of medications (OR 1.7-2.3). When controlling for comorbidity, reduced global health (OR = 1.5 (1.1-2.0)) and use of alternative health care (OR = 1.7 (1.3-2.4)) remained associated with IBS. Severity of abdominal pain/discomfort was a predictor of having to seek a physician for IBS (OR = 1.3 (1.2-1.5)).Conclusions. Symptoms of IBS were reported by 8% of Norwegian adults and had resulted in consultations with physicians for the majority in the long run. Subjects with IBS in the community were characterized by frequent somatic and psychiatric comorbidity. Their observed reduced health, working disability and increased use of health resources were largely explained by comorbid symptoms and disorders.