This study involves a two-year longitudinal investigation of adolescent participation in unstructured youth recreation centres and the development of antisocial behaviour. Participants were 1163 adolescents who represented 92% of all eighth-grade students in the town of Orebro, Sweden, during the fall of 1999. Antisocial young people and those with poor relations to parents or school were likely to become involved in the centres. After controlling for these selection influences, the frequency of youth centre participation was associated with a significant increase in antisocial behaviour over time for boys and girls. Youth centres that aggregated many antisocial peers together were particularly likely to promote the antisocial behaviour of new attendees. The findings are consistent with prior theory and research on youth development and out-of-school activities. Activities that lack structure and skill-building aims appear to attract high-risk adolescents and the resulting social environment is conducive to the development of antisocial behaviour. (Contains 3 tables, 2 figures and 1 footnote.)