Background: A limited number of mostly cross-sectional studies have examined the possible effects of power sports on aggressive and antisocial involvement in children and youth. The majority of these studies have serious methodological limitations, and results are partly contradictory. Longitudinal studies with representative, reasonably large samples and adequate dependent variables are lacking. Methods: The relationship between participation in power or fight and strength sports (boxing, wrestling, weightlifting, and oriental martial arts) and violent and antisocial behaviour was examined in a sample of 477 boys, aged 11 to 13 years at Time 1, over a two-year period. Making use of information about different participation patterns over time, the longitudinal design provided an opportunity to examine specified hypotheses about possible causal effects of power sports. Results: The total pattern of results strongly suggests that participation in power sports actually leads to an increase or enhancement of antisocial involvement in the form of elevated levels of violent as well as non-violent antisocial behaviour outside sports. In addition, there were no indications of selection effects; the presence of such effects would imply that boys who started with power sports were characterised by already elevated levels of antisocial involvement. Conclusions: The results provide strong support for the "enhancement hypothesis". The negative effects in boys seemed to stem from both the practice of power sports itself and from repeated contact with "macho" attitudes, norms, and ideals. The negative effects of participation in power sports represent a societal problem of considerable dimensions which has been largely neglected up to now.