Background - Little is known about the comparability of outpatients with irritable bowel syndrome (IBS) and patients with IBS in primary care with regard to severity of complaints, perceived limitations, other aspects of the complaints, and sex differences. Aims - To compare outpatients with lBS with primary care patients with IBS. Patients - One hundred and nine patients with IBS were recruited from general practices in Amsterdam and 86 patients with IBS were recruited from the outpatient clinic of the Department of Internal Medicine of the University Hospital in Nijmegen. Methods - Each patient completed a questionnaire on demographic variables, abdominal complaints, related complaints, and attributed causes of their abdominal complaints. The scores of the two groups were compared by univariate and multivariate analysis. Results - The outpatient group contained significantly more men, reported more severe abdominal pain, more frequent complaints, more interference with daily activities, and a higher degree of avoidance of activities (p<0.01) than the primary care group. When each sex was analysed separately, these differences remained for female (p<0.01) but not for male patients. Outpatients were more likely to attribute their complaints to somatic causes (p<0.01), whereas primary care patients were more likely to attribute their complaints to stress (p<0.01) or their agitated way of life (p<0.05). Multivariate analysis showed that a high severity score, a large number of additional complaints, and a low score on the stress attribution were important determinants for being in the outpatient group. Conclusions - Female outpatients consider their complaints to be more serious and interfering than do patients with IBS in primary care. Male outpatients were comparable to primary care patients with IBS. More research needs to be done into sex specific differences in lBS and into the factors that influence the decision to refer a patient with IBS.