Background: A natural and cheap way of increasing children's physical activity is stimulating unstructured outside play.Purpose: This study examined whether characteristics of the family and perceived physical environment were associated with the duration of children's outside play.Methods: Parents participating in the " Be Active, Eat Right" cluster RCT control group (N = 2007) provided information on potential predictors of outside play (i.e. family and perceived physical environment) of their 5-year-old child by questionnaire. Child outside play was assessed by parental reports both at five and seven years. Linear regression analyses, adjusted for seasonality, were performed to evaluate associations between potential predictors and child...