Asthma and allergic rhinitis often coexist and are increasing worldwide, particularly among the younger generation. Although the prevalences of adult asthma and allergic rhinitis and their risk factors have been reported, there have been few studies focusing on young adults. The aim of this study was to evaluate the prevalences of asthma and allergic rhinitis and their associated factors in Japanese young adults. A questionnaire survey of new students at Hokkaido University about the presence of current wheeze and rhinitis and a history of several viral infections during childhood was conducted in 2008 and 2010. The prevalences of wheeze and rhinitis and their associated factors were evaluated. Of 4076 nonsmoking subjects aged 18-25 years, 261 (6.4%) had current wheeze and 1373 (33.7%) had allergic rhinitis. On multivariate analyses, current wheeze was associated with high body mass index (BMI), atopic dermatitis, allergic rhinitis, food allergy, and a history of measles infection. In contrast, allergic rhinitis was associated with low BMI, current wheeze, atopic dermatitis, food allergy, and no history of measles. When subjects were classified into four groups by the presence or absence of wheeze and rhinitis, both high BMI and a history of measles were positively associated with wheeze without rhinitis but negatively associated with rhinitis without wheeze. High BMI and past measles infection showed contrasting associations with asthma and allergic rhinitis in nonsmoking young adults. It is important to not only recognize the common pathophysiological characteristics of asthma and allergic rhinitis but also to understand their differences.