To compare adolescent cannabis use between 2002 and 2006 and to investigate links to the frequency of evenings spent out with friends.The Health Behavior in School-Aged Children study, an international study carried out in collaboration with the World Health Organization/Europe.A total of 31 mostly European and North American countries and regions.A total of 93 297 students aged 15 years.Cannabis use in the last 12 months in relation to the mean frequency of evenings out with friends per week.A decrease in the prevalence of cannabis use was found in most of the 31 participating countries and regions. The most marked decreases were found in England, Portugal, Switzerland, Slovenia, and Canada. Increases occurred only in Estonia, Lithuania, Malta, and among Russian girls. The more frequently adolescents reported going out with their friends in the evenings, the more likely they were to report using cannabis. This link was consistent for boys and girls and across survey years. Across countries, changes in the mean frequency of evenings spent out were strongly linked to changes in cannabis use.The findings are consistent with the hypothesis that by going out less frequently in the evenings with friends, adolescents had fewer opportunities to obtain and use cannabis. Future research is needed to learn more about the nature of evenings out with friends and related factors that might explain changes in adolescent cannabis use over time.