Although irritable bowel syndrome (IBS) is a common disorder among gastrointestinal clinic outpatients, it continues to be a diagnosis of exclusion. In treatment-seeking populations, IBS has been frequently associated with psychiatric illness, and this co-occurrence has added to controversy about the validity of the IBS diagnosis. This study is a preliminary effort to examine the nature of this relationship by using the family study design. The probands consisted of 20 patients with IBS and 20 patients who had undergone laproscopic cholecystectomy. Their first-degree relatives were interviewed to obtain lifetime diagnoses of functional gastrointestinal and psychiatric syndromes. Significantly more IBS probands had lifetime psychiatric illness than the cholecystectomy probands. The lifetime prevalence of IBS as well as other functional gastrointestinal syndromes was not significantly different between the groups of relatives. However, significantly more relatives of the IBS probands had lifetime psychiatric illness than the relatives of the cholecystectomy probands. Among the relatives with functional gastrointestinal disorders, significantly more had psychiatric illness. This preliminary study provides support for a relationship between IBS and psychiatric illness by the finding of an increased prevalence of psychiatric disorders among the relatives of patients who have IBS.