Irritable bowel syndrome (IBS) runs in families. Prior family studies surveyed patients inquiring about family history without surveying family members. The stigma associated with IBS may lead relatives to not share information with others,resulting in underestimates of familial aggregation of IBS. The aim of the study was to evaluate the accuracy of patient-report of family history of IBS in cases and controls, and to estimate familial aggregation of IBS using both a case-control and a family-study design. Fifty cases and 53 controls completed symptom questionnaires and provided contact information for first-degree relatives. Questionnaires were mailed to relatives. Relatives were considered to have IBS if they met Rome criteria and did not have an alternate GI diagnosis. Cases and controls identified 573 relatives in their families. A total of 202 (51%) of 396 living relatives participated. The kappa statistics between proband- and relative-reported IBS for case- and control-relatives were 0.27 and 0.04. Cases reported 21%of relatives had IBS; relative-reports showed 37%(P = 0.003). Controls reported 4% of relatives had IBS;relative-reports showed 16% (P = 0.013). Regardless of whether the proband or the relative themselves were the information source, case-relatives were three fold as likely to have IBS than control-relatives (P < 0.05).However, overall rates were higher when data collected from relatives were used. Regardless of approach, strong familial aggregation of IBS was observed. Cases and controls underestimated the frequency of IBS in their relatives and agreement between proband- and relative-report of IBS status was extremely poor, thus emphasizing the need for direct data collection from relatives in IBS family studies.