Smoking habits of inflammatory bowel disease (IBD) patients were compared with family members with IBD to determine the effects of cigarette smoking on the development of either ulcerative colitis or Crohn's disease. In a previous study of patients randomly selected from the University of Chicago IBD Registry, 40 patients with IBD were identified as having a confirmed IBD family history. Thirty-one of those probands and relatives were contacted and included in this study. Of the 36 patients in the study with Crohn's disease, 78% were former or current smokers, whereas only 27% of the 26 patients with ulcerative colitis were former or current smokers. Eighty-three percent of all those who had ever smoked, started before the onset of symptoms of their disease. There were high concordance rates for type of IBD (81%) and for smoking habits (65%, when a history of smoking was compared with a history of never having smoked) among the matched pairs. The concordance rates were not significantly different from each other, but the magnitude of concordance of each suggests that both smoking habits and consanguinity may be important factors in the pathogenesis of this disease.