Mobile phones have become an essential part of the daily lives of teenagers in recent times. There are growing concerns among many Japanese people, including policy makers and the press, about the adverse effects of mobile phone use on adolescents. For example, the National Policy Agency (NPA) indicated the association between mobile phone use and juvenile delinquency. This study focuses on how use patterns and motivations are associated with the negative effects of mobile phones on the basis of a survey of junior high school students aged 14 years in Tokyo (N = 311). The factor analysis of motivations yields two factors, namely, emotionality and instrumentality. The first factor, emotionality, is significantly correlated with the frequency of mobile e-mailing and delinquency score. However, despite the significant and positive correlation between the delinquency score and frequency of mobile e-mailing, the effect of mobile e-mailing on delinquency and school grades is not significant when controlling for the motivation factors. These results show that the adverse effects of mobile phones are not supported.