Objective: To investigate school-aged children's and parents' attitudes, social influences, and intentions toward excessive screen-related sedentary behavior (S-RSB). Design: A cross-sectional study using a survey methodology. Setting: Elementary schools in London, Ontario, Canada. Participants: All grades 5 and 6 students, their parents, and their teachers in the participating schools were invited to voluntarily participate; 508 student-parent pairs completed the surveys. Main Outcome Measure: Children's screen-related behaviors. Analysis: Data were analyzed using the Independent Student t test to compare differences of continuous variables and the chi-square test to test for differences of categorical variables. Results: Children spent 3.3 [plus or minus] 0.15 (standard error) hours per day engaged in screen-related activities. Entertainment, spending time with family, and boredom were cited as the top 3 reasons for television viewing and video game playing. Compared to "low-screen users" (ie, less than 2 hours/day), "high-screen users" (ie, [greater than or equal to] 2 hours/day) had a less negative attitude toward excessive S-RSB and perceived loosened parental rules on screen use. Parents of high-screen users had a less negative attitude toward children's S-RSB, had fewer rules about their children's screen use, and were more likely to be sedentary themselves. Conclusions and Implications: Intervention strategies aimed at reducing S-RSB should involve both parents and children and should focus on fostering behavioral changes and promoting parental role modeling. (Contains 3 tables and 1 figure.)