Previous research by Kandel [1] and others indicates that adolescent drug use follows a progression from legal drugs, through marijuana, to hard drugs. In this study of drug use patterns, institutionalized delinquents were found to follow a similar progression of drug use. Unlike previous studies which used "rule of thumb" methods of model assessment, this study uses probabilistic assessment of the models. Importantly, the present study supports a modified gate-way sequence, where cocaine use appears as an intermediate step between marijuana use and use of other hard drugs. It is suggested that widespread availability of cocaine in the late 1980's may have resulted in a new "step" in the drug use sequence.