This study explored the effects of parental monitoring, leisure boredom, and leisure activity on Internet addiction. The sample was 1,289 adolescents from eleven senior high schools in Taiwan. Participants were asked about their perception of being monitored by their parents, leisure boredom, leisure activities, and Internet addiction behavior. Results showed that leisure boredom and involvement in Internet and social activities increase the probability of Internet addiction; however, family and outdoor activities along with participative and supportive parental monitoring decrease these tendencies. Overall evidence suggests that parental monitoring is a major inhibitor of Internet addiction. Thus, adolescents should be supervised in their daily routines and encouraged to participate in family and outdoor activities. In addition, adolescents should develop a positive attitude toward leisure and the skills to prevent overdependence on online relationships with the assistance of parents. These findings suggest the preventive strategies regarding Internet addiction.