Multivariate regression analyses were performed on data obtained from about 61,000 high school seniors, 88,000 8th grade students, and 82,000 10th grade students. These data were collected from the Monitoring the Future project, an ongoing nationwide study of youth conducted under a series of research grants from the National Institute on Drug Abuse in which students completed self-administered questionnaires. The specific goal of the current study was to determine whether recent increases in marijuana use among high school seniors and among students in earlier grades reflected similar processes. Results showed individual lifestyle factors (grades, truancy, religious commitment, and evenings out for recreation) correlated substantially with marijuana use but did not explain changes in marijuana use over time. Rather, decreases in perceived risk of harmfulness and in disapproval accounted for recent increases in all three grades and for earlier decreases among high school seniors. The authors conclude that perceived risks and disapproval are important determinants of marijuana use and that prevention efforts should include realistic information about risks and consequences of marijuana use. 17 references, 2 tables, and 1 figure