Background/Aims: Familial Crohn’ s disease has shown concordance concerning location and clinical type of the disease especially among monozygotic twins. Susceptibility to Crohn’ s disease is both based on genetic and environmental factors. We investigated polymorphisms of CARD15, TLR4, and OCTN, and environmental factors in a monozygotic twin pair with Crohn’ s disease and their first- degree relatives. Methods: 22- year- old monozygotic female twins with ileocolonic Crohn’ s disease and their healthy brother and parents were examined. DNA samples from patients and relatives were genotyped for CARD15, TLR4, and OCTN polymorphisms. ASCA and pANCA analyses were performed. Additionally, patients and relatives filled out a questionnaire concerning multiple environmental factors. Results: Both twins presented in the same year with identical Vienna Classification phenotypes: stenotic behavior (B2) and localization in terminal ileum and colon (L3). Both carried a CARD15 R702W variant, but had normal alleles in TLR4 and OCTN. They were smokers since the age of 15, used oral contraceptives and had undergone appendectomy. The healthy father and brother were CARD15 R702W positive, were non- smokers and had not undergone appendectomy. Conclusion: This case report is the first to describe complete concordance in CARD15 status, phenotypic appearance, and smoking, appendectomy and oral contraceptive use in a pair of monozygotic twins with CD. Background / Aims: Familial Crohn’s disease has shown concordance concerning location and clinical type of the disease in among the monozygotic twins. Susceptibility to Crohn’s disease is based on genetic and environmental factors. We investigated polymorphisms of CARD15, TLR4, and OCTN , and environmental factors in a monozygotic twin pair with Crohn’s disease and their first-degree relatives. Methods: 22- year-old monozygotic female twins with ileocolonic Crohn ’s disease and their healthy brother and parents were examined. DNA samples from patients and relatives were genotyped for CARD15, TLR4, and OCTN polymorphisms. ASCA and pANCA analyzes were performed. Results: Both twins presented in the same year with identical Vienna Classification phenotypes: stenotic behavior (B2) and localization in terminal ileum and colon (L3). Both carried a CARD15 R702W variant , but had normal alleles in TLR4 and OCTN. They were smokers since of age of 15, used oral contraceptives and had undergone appendectomy. The healthy father and brother were CARD15 R702W positive, were non- smokers and had not undergone appendectomy. Conclusion: This case report is the first to describe complete concordance in CARD15 status, phenotypic appearance, and smoking, appendectomy and oral contraceptive use in a pair of monozygotic twins with CD.