Our objective was to determine whether there is a greater concordance of interstitial cystitis (IC) among monozygotic than dizygotic twins. Members of the Interstitial Cystitis Association (ICA) who responded to a survey about first-degree family members with IC symptoms or confirmed IC were requested to identify themselves if they were 1 of a twin pair. Each twin respondent and co-twin were then evaluated via a questionnaire and acquisition of hydrodistention reports as to their meeting modified National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) criteria for IC. Of the co-twins of 8 monozygotic twin respondents, 2 had probable and 3 had confirmed IC compared with none of the co-twins of the 26 dizygotic twin respondents (including 15 female co-twins). There is a greater concordance of IC among monozygotic than dizygotic twin pairs suggesting a genetic susceptibility to IC.