We investigated the role of physical characteristics of kindergartens' outdoor play areas in teacher-rated physical aggression (PAgg) among 423 children followed annually from ages two to four years. We used data from the Behaviour Outlook Norwegian Developmental Study which follows children from southeast Norway, a country where almost all two- to four-year-old children attend kindergartens. Nesting children in kindergartens, we found two significant associations after adjustment for family selection. First, children in kindergartens with more 'secret places' in their outdoor play areas (where they could play undisturbed) had more PAgg at baseline. Second, children in kindergartens with more adult supervision of their use of outdoor play material showed a less steep decrease in PAgg over time. If causal, these associations would suggest that children in kindergartens should not play completely unmonitored but also that teachers should not control children's outdoor play excessively.