The study found that a higher neighborhood concentration of single-mother families decreased programs effectiveness, as did neighborhood poverty. High immigrant composition of neighborhoods, on the other hand, increased program effectiveness. An unexpected finding was that the program was also more effective in neighborhoods with higher crime rates. Aside from this latter finding, the other findings are consistent with theories of social disorganization, immigrant adaptation, and social isolation. Whites and more linguistically acculturated Latinos showed no difference in program effectiveness by neighborhood characteristics. Study data came from a randomized trial of a substance-use prevention program in Phoenix, AZ, called "keepin' it REAL." The program was implemented in 35 middle schools beginning in the fall of 1998. The sample of 4,622 middle-school students was composed primarily of Mexican-Americans. Prior to implementation of the prevention program, students in all the participating schools completed a pretest survey that measured the adolescents' experiences with substance use norms and family and individual background characteristics. The assignment of schools to treatment or control conditions was done through block randomization that controlled for the size and ethnic composition of the schools. In the late spring of 1999, a follow-up questionnaire was administered approximately 2 months after delivery of the prevention programs was completed in treatment schools. This survey repeated many of the measures in the pretest survey. This survey provided evidence of any change related to program participation. The dependent variable in the analysis was recent alcohol use. The independent variables were neighborhood characteristics determined from the 2000 U.S. Census. 4 tables and 64 references