The relationship between dropping out of high school and substance use was explored using the 1984 National Longitudinal Survey of Young Adults. Relevant research literature is reviewed. Cross-sectional data indicated that high school dropouts were more involved with cigarettes and illicit drugs than were graduates and that those who obtained a graduate equivalency diploma were the most intensively involved. Event history analysis indicated that prior use of cigarettes, marijuana, and other illicit drugs increases the propensity to drop out and that the earlier the initiation into drugs, the greater the probability of premature school leaving. Thus, preventing or at least delaying the initiation of drug use will reduce the incidence of dropping out of high school. These findings have broad implications for the development of intervention strategies for at risk students. Statistical data are presented on seven tables. A list of 58 references is included. (VM)