Purpose: In Korea, there have been many epidemiologic studies about allergic diseases of school children, but so far only a few studies of preschool children have been done. Research on preschool children will be a great help to understand the epidemiology of the allergic diseases in the pediatric population. In this perspective, we researched the prevalence and risk factors of allergic diseases in preschool children in Jung-gu, Seoul. Methods: A modified International Study of Asthma and Allergies in Childhood questionnaire survey was done on 917 (male, 492; female, 425) preschool children in Jung-gu area of Seoul in June, 2009. Parents or caregivers answered the questionnaires that asked about the prevalence of allergic diseases and the risk factors. Results: For asthma, the prevalence of "wheeze, ever", "diagnosis" and "treatment, last 12 months" were 18.4%, 5.8% and 2.8%, respectively. For allergic rhinitis (AR), the prevalence of "rhinitis, ever", "diagnosis" and "treatment, last 12 months" were 31.8%, 16.2% and 12.9%, respectively. For atopic dermatitis (AD), the prevalence of "itchy rash, ever", "diagnosis" and "treatment, last 12 months" were 26.1%, 34.3% and 19.0%, respectively. The risk factors of asthma were cesarean section delivery, use of antibiotics and history of bronchiolitis before the age of 2 years. The risk factor of AR was history of asthma. The risk factor of AD was parent allergy. Conclusion: In comparison to preceding studies of preschool children in Seoul, the prevalence of allergic diseases is similar. In comparison to the studies of school children, the prevalence of AR was lower and the prevalence of AD was higher. [Pediatr Allergy Respir Dis(Korea) 2011;21:165-175]