Asthma and allergic diseases are one of the most common disorders in children. Due to its increased prevalence, as well as the increased morbidity and mortality from these diseases, asthma and allergic diseases have come to be recognized as a major worldwide public health issue. In addition, socioeconomic burden of asthma and allergic diseases has increased in Korea also. The International Study of Asthma and Allergies in Childhood (ISAAC) investigated the worldwide prevalence of asthma and other allergic diseases using simple standardized methods because of the comparison of asthma and allergic diseases between the countries. In Korea, several epidemiologic studies have been conducted to determine the prevalence of asthma in children. Although these studies showed increased prevalence of asthma among Korean children (from 3.4% in 1964 to 10.1% in 1989), these findings were based on data from small numbers of subjects. The first Nationwide Study of Asthma and Allergies in Korean Children, which utilized the Korean version of ISAAC written and video questionnaire, was conducted in 1995 and the second Nationwide Study of Asthma and Allergies in Korean Children was conducted in 2000, directed by the Korean Academy of Pediatric Allergy and Respiratory Diseases. We report here the prevalence of asthma and other allergic diseases in Korean children and adolescents, and show the changes that occurred over this 5 year period. We also describe the risk factors for development of these diseases in Korean children. We developed Korean versions of the ISAAC written (WQ) and video (AVQ) questionnaires for allergic diseases. In 1995, the enrolled population consisted of 25,117 children selected from 34 elementary school and 14,946 children selected from 34 middle school across the nation, the response rate was 94.8%. In 2000, 27,831 children selected from 34 elementary school and 15,214 children selected from 34 middle school, and the response rate was 96.4%. From these studies, we can confirm that increase of the prevalences of asthma, allergic rhinitis, and atopic dermatitis in Korea, except the prevalence of food allergy. Especially, the video questionnaire showed increases in the lifetime and 12 month prevalence rates of wheeze at rest, exercise-induced wheeze, nocturnal wheeze, nocturnal cough, and severe wheeze over this period of time in middle school children. In addition, the increase of prevalences of symptoms, diagnosis and treatment of atopic dermatitis was noted significantly. Risk factor analysis showed that body mass index (BMI), passive smoking and living with a dog or cat were associated with higher risk of wheeze. Also the occurrence of fever during infancy and the frequent use of antibiotics were associated with the risk of wheeze. In conclusion, during the 5 year period from 1995 to 2000, the prevalences of asthma, allergic rhinitis, and atopic dermatitis has increased in Korean children. BMI, passive smoking, living with a dog or cat, the fever episodes in infancy, and the frequent use of antibiotics in infancy are important risk factors to development of asthma and atopic dermatitis. In the near future, the birth cohort study will be needed to investigate the causes of this increase and the natural course of allergic diseases, then we develop the methods to control asthma and allergic diseases.