Whether or not to decriminalize the possession of small amounts of marijuana has been one of the most hotly debated policy issues to come onto the U.S. scene over the last decade. This study sought to determine whether decriminalization has had an effect on marijuana use in high school seniors and young adults. In 10 states, marijuana possession has been decriminalized. Using most of the remaining states as a control group, the study attempted to assess the impact of the law on the attitudes, beliefs, and peer norms of high school seniors and young adults relating to marijuana use. Two data sources were used: repeated cross sections from successive classes of graduating high school seniors and longitudinal data on several panels of individuals drawn from the high school graduating classes of 1975 and 1976. Overall, the results indicated that decriminalization of marijuana had virtually no effect either on use or on related attitudes and beliefs about marijuana use among young people in this age group. (BHK)