Objective: Communities Mobilizing for Change on Alcohol (CMCA) was a randomized 15-community trial of a community organizing intervention designed to reduce the accessibility of alcoholic beverages to youths under the legal drinking age. Method: Data were collected at baseline before random assignment of communities to intervention or control condition, and again at follow-up after a 2.5-year intervention. Data collection included in-school surveys of twelfth graders, telephone surveys of 18- to 20-year-olds and alcohol merchants, and direct testing of the propensity of alcohol outlets to sell to young buyers. Analyses were based on mixed-model regression, used the community as the unit of assignment, took into account the nesting of individual respondents or alcohol outlets within each community, and controlled for relevant covariates. Results: Results show that the CMCA intervention significantly and favorably affected both the behavior of 18- to 20-year-olds (effect size = 0.76, p < .01) and the practices of on-sale alcohol establishments (effect size = 1.18, p < .05), may have favorably affected the practices of off-sale alcohol establishments (effect size = 0.32, p = .08), but had little effect on younger adolescents. Alcohol merchants appear to have increased age-identification checking and reduced propensity to sell to minors. Eighteen- to 20-year-olds reduced their propensity to provide alcohol to other teens and were less likely to try to buy alcohol, drink in a bar or consume alcohol. Conclusions: Community organizing is a useful intervention approach for mobilizing communities for institutional and policy change to improve the health of the population.