We report results of a prospective examination of the influence of outcome expectancy variables and inherited temperaments on the onset of drinking over a 12-month period in a sample of 1,164 high school students. While univariate prospective analysis indicated that drinkers and nondrinkers were different both on measures of outcome expectancy and temperament, multivariate analysis supported, most strongly, a social learning account of the processes influencing the onset and maintenance of drinking behavior in this sample. The multivariate analysis revealed that only expectancies for enhanced social behavior were consistently associated with the onset of drinking from baseline to 12-month follow-up ( p < .001). Among all nondrinkers at baseline, those entertaining higher expectancies about the positive effects of alcohol on social interaction were more likely to begin drinking between baseline and follow-up. At present, few, if any, alcohol abuse prevention studies with adolescents have explicitly attempted to alter alcohol expectancies or to establish a link between expectancy and behavior change. Our results suggest that it may be useful to do so.