Aim: The purpose of this study was (1) to examine the changes in participation rates over 1 year among children and adolescents after acquired brain injury and (2) to explore the effect of child and family factors on these changes. Method: The participation levels of 136 children and young people (88 males; 48 females; age range 4y 11mo-17y 6mo; mean age 11y 6mo) after acquired brain injury (3 less than or equal to Glasgow Coma Scale score less than or equal to 15; mean 12.8) were assessed three times: at their return to school, and at 8 and 12 months after returning to school. The Children's Assessment of Participation and Enjoyment measured the participants' diversity and intensity of participation in out-of-school activities. At baseline, information on general family functioning and medical and demographic information was collected as possible predictors. Mixed-effect model analyses of participation scores were performed while controlling for child's age at injury. Results: The severity of the injury explained rates of change across time for participation intensity in recreational, physical, and social activities. Household income influenced changes in the intensity of recreational activities, whereas family functioning predicted changes in the diversity of skill-based activities. Interpretation: Participation is a relevant outcome of recovery that needs to be assessed and monitored post brain injury. Special attention can be directed to severity of injury and family functioning when developing intervention plans.