The purpose of this study was to examine whether physical activity moderates the relationship between school related stress and subjective health complaints in a representative sample of adolescents. The data material was based on an international study on "Health Behaviours in school-aged children" (the HBSC study) conducted in collaboration with the World Health Organisation. The data were collected in December 1997 and include a national representative sample of 1670 Norwegian 15 year olds. Analysis were performed by hierarchical multiple linear regression analysis. The results suggested that physical activity have a significant moderating effect on the relation between subjective health complaints and school related stress, indicating that physical activity may alleviate the harmful effects of school related stress.