This study evaluates the effects of a school-based smoking prevention program after 1 year, using school (22 middle/elementary schools, 15 high schools) as both the unit of randomization and the unit of analysis. The multigrade level (grades 6 through 9) intervention was designed to address comprehensively the social influence factors that encourage smoking. Teacher survey data indicated that treatment schools had a median of 10 classroom sessions devoted to tobacco/drug use education, 5 of which were the sessions designed for this evaluation, and control schools had also dedicated a median of 10 classroom sessions to tobacco/drug education. Thus, the study evaluated the incremental effects of the social influence intervention compared to "standard-care" curricula. Among those who reported smoking one or more cigarettes in the month prior to the intervention, there was a significant treatment effect on rate of smoking at one year, but no grade level, gender, or interaction effects. The 1-year covariate-adjusted smoking rate among pretest smokers in the treatment schools was 76.6 cigarettes per month, compared to 111.6 cigarettes per month in control schools, a 31.4% difference. These effects were not accounted for by differential subject attrition. The analyses for nonsmokers, however, showed no significant effects, and the program did not affect self-reported alcohol or marijuana use. Taken together with the results of other prevention studies, these results point to the need for the development and evaluation of new initiatives to prevent substance use.