Marijuana use among high school seniors has been declining since 1979. This paper explores two alternative explanations of this trend: that young people have become more conservative in general, or that specific changes in views about marijuana have led to the decline in its use. We report bivariate and multivariate analyses of questionnaire data from annual nationwide surveys of high school seniors, 1976 through 1986. Lifestyle factors such as religious commitment, truancy, and evenings away from home are linked strongly to individual differences in marijuana use: these factors, however, have not trended in ways that can account for much of the recent decline in marijuana use. On the other hand, the analyses suggest that if perceived risks and disapproval associated with regular marijuana use had not risen substantially in recent years, the decline in actual use would not have occurred. The implications for drug use prevention efforts are discussed.