The basic research design for the study was a cohort-sequential design in which graduating classes were followed over time. Beginning in 1975, a nationally representative sample of each year's senior class in public and private high schools in the coterminous United States was surveyed. Results are presented in four sections: (1) An Assessment of Change in Use by Seniors; (2) An Assessment of Change in Use by Panels of Seniors from the Classes of 1975-1976; (3) Assessing the Impact of Decriminalization on Possible Intervening Variables; and (4) Trends in Knowledge of the Prevailing Law. The preponderance of the evidence gathered and examined for this study points to the conclusion that decriminalization had virtually no effect either on the marijuana use or on related attitudes and beliefs about marijuana use among American young people in this age group. The degree of disapproval young people hold for marijuana use, the extent to which they believe such use is harmful, and the degree to which they perceive the drug to be available to them were also unaffected by the law change. Notes, figures, tables, references, appendixes