This Canadian project is an ecological and participatory approach developed to prevent alcohol and other drug use and misuse among multiethnic youth. The intervention was implemented through four channels of program delivery: families, schools, community organizations, and local government. The study involved 411 sixth graders from 8 elementary schools and 380 eighth graders from 2 junior high schools in 2 school districts of the Island of Montreal (Province of Quebec). All students were enrolled in regular classes. Follow-up data were collected 18 months and 30 months after a pretest, using a school survey. The findings showed that the program had no significant impact on alcohol use. The program was, however, capable of producing a significant effect on several hypothesized mediating variables. At first follow-up, the sixth graders showed a higher self-esteem, better peer-pressure resistance skills, and a more positive relationship with their fathers than the controls. The eighth graders were also more inclined to get involved in community activities related to substance abuse prevention and to choose more alternatives to substance abuse in their leisure time than the controls. These results are discussed by examining attrition effects and also reasons for program failure. Issues in the evaluation of an ecological and participatory approach are also addressed. 6 tables, 1 figure, and 53 references