PURPOSE: It has been suggested that caesarean section is a risk factor for allergic diseases under the 'hygiene hypothesis', but it remains controversial. In this study, we examined the association between mode of delivery and development of allergic diseases. METHODS: The Korean International Study of Asthma and Allergies in Childhood questionnaire was used to survey 8,404 elementary and middle-school students in Seoul between April and October 2008. We categorized children into a caesarean section group and a vaginal delivery group, and surveyed the prevalence of asthma, allergic rhinitis, and atopic dermatitis. RESULTS: No differences in the prevalence of "wheeze ever" (P=0.418), "wheezing the last 12 months" (P=0.152), and "diagnosis of asthma ever" (P=0.382) were observed between the caesarean section group and the vaginal delivery group. The prevalence of "rhinitis ever" (P=0.609), "rhinitis the last 12 months" (P=0.788), and "diagnosis of allergic rhinitis ever" (P=0.700) was also similar between the two groups. The prevalence of "itchy eczema ever" (P=0.065), "itchy eczema last 12 months" (P=0.381), and a "diagnosis of atopic dermatitis ever" (P=0.162) also did not show any differences between the two groups. However, in subjects with a family history of allergic disease, the caesarean section group had the higher prevalence of "diagnosis of asthma ever" (11.3% vs. 9.2%, P=0.032) than that in the vaginal delivery group. CONCLUSION: We found an association between mode of delivery and the subsequent development of asthma in children with a family history of allergic disease in Seoul, Korea.