Previous studies have shown an increased risk of inflammatory bowel disease (IBD) among relatives of patients with Crohn disease and ulcerative colitis. In the present study the probandwise concordance rates for ulcerative colitis and Crohn disease among mono- and dizygotic twins were estimated. Further we aimed to evaluate whether smoking habits might influence the concordance, and to look for clinical characteristics of concordant versus discordant twin pairs. Among the 38,507 identified twins born in Denmark from 1953 to 1982, a questionnaire was sent to the 34,076 who previously had accepted to participate in studies. For twins reporting IBD, the diagnosis was verified by applying standard criteria to records requested from hospitals or practitioners. Among the 29,421 (86.3%) twins answering the questionnaire, 103 pairs had at least one twin who suffered from IBD. In the Crohn disease group five of 10 monozygotic pairs, but none of 27 dizygotic pairs were concordant. In the ulcerative colitis group three of 21 monozygotic, and two of 44 dizygotic pairs were concordant. The probandwise concordance rate among monozygotic pairs was 58.3% for Crohn disease and 18.2% for ulcerative colitis; among the dizygotic pairs the rates were 0 and 4.5%, respectively. The frequency of smokers was higher among twins with Crohn disease and lower among twins with ulcerative colitis compared to the frequency in the twin register. Furthermore, smoking habits were found to be of significance for discordance for disease. Regarding the clinical characteristics no homogenous pattern was observed within the concordant pairs and the differences between concordant and discordant pairs were not significant. The observation of a significantly higher concordance rate among monozygotic than among dizygotic twin pairs strongly points to a genetic influence on occurrence of IBD, which seems to be more pronounced with regard to Crohn disease than to ulcerative colitis. Differences in smoking habits among the members of the discordant twin pairs may influence the discordance.