The current article provides an important insight into measurement differences between two commonly used self-reports and accelerometer-determined moderate to vigorous physical activity (MVPA) scores within matched samples across 1 school year. Participants were 998 fifth- through eighth-grade students who completed self-reports and 76 fifth- and sixth-grade children with accelerometers. The Health Behavior in School-aged Children (HBSC) Research Protocol showed the higher frequency of days exceeding 60 minutes of MVPA than the International Physical Activity Questionnaire Short Form (IPAQ-SF). Larger proportions of children achieved the MVPA guidelines based on HBSC results (girls 16%, boys 29%), than IPAQ-SF (girls 5%, boys 11%), or accelerometers data (girls 0%, boys 0%). MVPA remained stable over 12 months on the basis of both self-report and accelerometer results. Both self-report scales presented should be used with discretion in population studies, whereas accelerometer-determined data can be more authentic when personal guidance is required.