The increasing prevalence of asthma has coincided with an increase of body mass index (BMI) in both children and adults. We investigated the relationship between BMI and the symptom prevalence of asthma and the possible influences of dietary pattern. This was a community-based, cross-sectional study of 24,260 school children aged 6-12 years. Prevalences of asthma and potential confounding factors were assessed using a Korean version of the International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire which was completed by parents. We analyzed the relationship between BMI and symptoms of asthma and the possible influences of dietary pattern. A significant positive association between high BMI and previous 12-month prevalence of wheeze remained in boys (adjusted odds ratio, 1.610; 95% confidence interval, 1.274-2.033) but not in girls. In addition, there were significant associations between high BMI and lifetime prevalence of wheeze, previous 12-month wheeze, exercise-induced wheeze, diagnosis, and treatment of asthma. There were significant associations between high BMI and previous 12-month wheeze regardless of breast-milk feeding or whole-milk feeding. Frequent intake of fresh seafood, fresh fruits, and vegetables was associated with reduced prevalence of current asthma symptoms and was also associated with decreased BMI. These results indicate that BMI may be an independent risk factor for the development of asthma symptoms in boys. Intake of fresh seafood, fresh fruit, and vegetables, which may be associated with decreased BMI, may contribute to protect against the development of asthma symptoms in Korean elementary schoolchildren.