PURPOSE: Pathological Internet use has been predominantly studied in junior high/middle school-aged or older children; data from elementary/primary school-aged children, however, are scarce. The current study aimed to examine the prevalence of problematic Internet use, including pathological and maladaptive Internet use, in elementary and junior high school-aged children and the relationships between problematic Internet use and mental health problems and health-related quality of life. METHODS: The survey was conducted among children who attend national and public elementary and junior high schools in a medium-sized city in Japan; data were received from 3845 elementary school-aged and 4364 junior high school-aged children. RESULTS: Based on the Young's Diagnostic Questionnaire score, the prevalence of pathological and maladaptive Internet use was 3.6% and 9.4% and 7.1% and 15.8% in elementary and junior high school-aged children, respectively. The prevalence of problematic Internet use, including pathological and maladaptive Internet use, consistently increased from the 4th grade to the 8th grade. In addition, the prevalence sharply increased between the 7th grade and the 8th grade. Our study revealed that children with pathological and maladaptive Internet use exhibited more severe depression and decreased health-related quality of life than those with adaptive Internet use. CONCLUSIONS: Our results demonstrated that pathological Internet use is not uncommon even in elementary school-aged children and that those with pathological and maladaptive Internet use have severe mental health problems and decreased health-related quality of life, supporting the importance of providing these children with educational and preventive interventions against problematic Internet use and associated risk factors.