Adolescents' religious involvement occurs within a social context, an understudied aspect of which is relationships with peers. This longitudinal study assessed changes in religiosity over 1 year and explored the extent to which these were associated with their friends' religiosity and problem behavior. The first year sample included 1,010 (52.5% female) Muslim 13 and 15 year old Indonesian adolescents; 890 of these were assessed 1 year later. Adolescents were similar to their friends in religiosity. Changes in religiosity from year one to two were associated with friends' religiosity such that adolescents with religious friends were more religious at year two than those with less religious friends. Reductions in religiosity were also associated with the presence of problem behavior, consistent with the inverse relationship between these. Peers may play an important role in the development of adolescent religiousness and exploring these influences deserves further study.