Background: Genetic predisposition as a cause of inflammatory bowel disease (IBD) has been proven by 
both family and twin studies and genetic variants associated with the disease have been identified. 
The aim of our study was to determine the concordance rates for IBD in German twin pairs and to 
evaluate clinical characteristics of concordant and discordant twin pairs.

Methods: Patients with IBD were asked to participate and complete a questionnaire that contained 
questions about zygosity, demographic data, and medical history.

Results: A total of 189 twin pairs in which at least 1 member had IBD were recruited (68 monozygotic 
and 121 dizygotic pairs). Within monozygotic pairs, 11 out of 31 (35%) were concordant for Crohn's 
disease (CD) and 6 out of 37 (16%) for ulcerative colitis (UC). Two of the 58 (3%) dizygotic pairs 
with CD and 1 out of 63 (2%) dizygotic pairs with UC were concordant for the disease. In 14 out of 
20 (70%) discordant monozygotic CD pairs and 25 out of 31 (81%) discordant monozygotic pairs with 
UC, the first-born was affected by IBD. For discordant dizygotic twins, the first in birth order had 
IBD in 33 out of 56 (59%) pairs with CD and 40 out of 62 (64.5%) pairs with UC.

Conclusions: This study confirms a stronger genetic influence in CD than in UC. The high 
preponderance in being affected of the first-born twin and the fact that concordance was only 35% 
for CD and 16% for UC monozygotic twins highlight the important role of environmental trigger 
factors.