Scholars have an enduring interest in investigating whether a new medium displaces or complements existing media whenever a new medium is introduced to the society. In this study, it is sought to investigate the extent to which the mobile phone would replace or reinforce teens’ traditional Internet use through computers. Surveys were conducted on 1875 youths between 12 to 17 years old in 5 digital cities in East Asia: Hong Kong, Seoul, Singapore, Taipei, and Tokyo. The findings profile a digital generation that is highly connected in the 21st century. Ninety percent of all respondents used the Internet. Almost 90% of the teens in this study used the mobile phone, and two-thirds of them ever surfed the Web via their mobile devices. Through a factor analysis, three dimensions of teens’ use of mobile Internet were identified, namely, task-based activities, information seeking and communication activities, and recreational activities. In general, the teens tend to use the mobile phone for recreation and entertainment purposes, especially playing games and listening to music. They are less likely to use the mobile phone for more sophisticated purposes, such as petitioning, voting, or shopping. Drawing upon the niche theory, the patterns between teens' mobile Internet and PC Internet use were further compared. Our analysis shows that mobile Internet primarily serves as an extension of teens’ Internet activities via PC, rather than as a replacement. Such positive relationships are quadratic and non-linear.