BACKGROUND & AIMS The rarity of inflammatory bowel disease (IBD) in both husband and wife is often given as an argument against an infectious origin. We registered conjugal instances of IBD in Northern France and in Belgium between 1989 and 2000. METHODS Couples were assigned to group A if both partners had symptoms of IBD before cohabitation, to group B if one spouse had IBD before cohabitation and the other experienced first symptoms afterwards, and to group C if both partners got the disease after cohabitation. Risk of IBD was assessed in their offspring. RESULTS Thirty conjugal instances were registered. Seventeen were concordant for Crohn's disease and 3 for ulcerative colitis; 10 were mixed. Two belonged to group A, 6 to group B, and 22 to group C. In group C, IBD occurred in the first affected spouse an average of 9 years after cohabitation and in the second spouse an average of 8.5 years later. Group C conjugal forms were more frequent than expected by chance (P < 0.02). Fifty-four children were born to 25 couples; among them 9, of whom 4 were siblings, developed Crohn's disease at a median age of 15 years. CONCLUSIONS The frequency of conjugal forms of IBD suggests an etiologic role for environmental factors. Offspring of 2 affected parents have a high risk of developing IBD.