The Community Youth Development Study (CYDS) is a community-randomized trial of the Communities That Care (CTC) prevention system. Using data from 2001 and 2004 administrations of the Community Key Informant Survey, this study reports changes in three community-level outcomes 1.5 years after implementing CTC in 12 communities. Respondents consisted of 534 community leaders in 24 communities representing multiple sectors within each community. Results of multilevel analyses controlling for respondent and community characteristics indicated that (a) CTC and control communities had comparable baseline levels of adopting a science-based approach to prevention, collaboration across community sectors, and collaboration regarding specific prevention activities; and (b) CTC communities exhibited significantly greater increases in these outcomes between 2001 and 2004 relative to control communities. These results suggest that CTC was successful in changing proximal system outcomes theorized to lead to more effective prevention services and, ultimately, reduced risk, enhanced protection, and improved adolescent health and behavior outcomes.