Obesity may increase the risk of subsequent asthma. We have previously reported that there is a clear association between obesity and asthma in Japanese school-aged children. To evaluate whether a similar association exists in younger children, a nationwide cross-sectional questionnaire-based survey was performed focusing on children aged 4-5 yr. A child who had experienced wheezing during the past 12 months and had ever been diagnosed with asthma by a physician was defined as having current asthma. Overweight and underweight were defined as BMI ≥90th percentile and ≤10th percentile, respectively, according to the reference values for Japanese children from 1978 to 1981. After excluding 2547 children because of incomplete data, 34,699 children were analyzed. Current asthma was significantly more prevalent in overweight children compared with underweight and normal weight children (13.2% for overweight vs. 10.5% for underweight and 11.1% for normal weight; both p < 0.001). Even after adjusting for other variables, such as gender, other coexisting allergic diseases, and parental history of asthma, there was an association between overweight and current asthma (adjusted odds ratio: 1.23, 95% CI: 1.10-1.38, p < 0.001). Even in preschool children, obesity is already associated with asthma, and there was no gender effect on this association. Physicians should consider the impact of obesity when managing asthma in younger children.