Although there are several symptom classification systems for irritable bowel syndrome (IBS), for example, the Manning or Rome criteria, there has been no previous research on how well these classifications correspond to each other or to persons diagnosed with IBS. We examined data from the Digestive Disorders Supplement of the 1989 National Health Interview Survey (NHIS) to assess demographic, socioeconomic, and health status characteristics of two groups of IBS sufferers--those who met the Manning or Rome criteria and those who reported having IBS but did not meet either criteria. The results showed that the overlap between estimates of IBS and persons with IBS-like symptoms depended on the definition of IBS. There were socioeconomic differences between the two IBS groups and higher rates of functional or work-related activity limitation and health care utilization for all IBS groups relative to US national averages. In summary, these findings indicate that IBS affects a large portion of the US population, regardless of the definition used to describe the condition. Our results suggest that there is a large undiagnosed population with numerous symptoms consistent with IBS, but further research is required to determine the differences between people who do and do not seek care, their impact on the health care system, and future therapies to reduce symptomatology and suffering.