AIM: To test how the quality of the outdoor environment of child day care centers (DCCs) influences children's health. METHODS: The environment was assessed using the Outdoor Play Environmental Categories (OPEC) tool; time spent outdoors, and physical activity as measured by pedometer. 172/253 (68%) of children aged 3.0-5.9 from nine DCCs participated in Southern Sweden. Health data collected were body mass index, waist circumference, saliva cortisol, length of night sleep during study, and symptoms and wellbeing which were scored (one-week diary- 121 parent responders). Also, parent-rated wellbeing and health of their child were scored (questionnaire, 132 parent responders). MANOVA, ANOVA and principal component analyses were performed to identify impacts of the outdoor environment on health. RESULTS: High quality outdoor environment at DCCs is associated with several health aspects in children such as leaner body, longer night sleep, better wellbeing and higher mid-morning saliva cortisol levels. CONCLUSION: The quality of the outdoor environment at DCCs influenced the health and wellbeing of preschool children and should be given more attention among health care professionals and community planners. © 2012 The Author(s)/Acta Paediatrica © 2012 Foundation Acta Paediatrica.