The study used data from the Careers in Crack Project, which included interviews with hard-drug abusers and sellers directly recruited from inner-city Manhattan during 1988-1989. After informed consent was obtained, subjects were asked about their initiation into the use of various drugs, especially crack; the social process of crack involvement; drug sales; and other nondrug criminality. Researchers used Markov models to identify common pathways through use of various drugs that led to eventual use and frequently abuse of crack. The drugs included in this analysis were "gateway" drugs, snorting cocaine, intravenous drugs use, and crack cocaine. The study found that the sequence of drug use was strongly mediated by birth year. Nearly all (81 percent) current crack abusers born prior to 1953 had previously injected heroin. At the other extreme, prior heroin injection was rare (10 percent) among crack abusers born since 1967. Many (37 percent) of these younger crack abusers initiated use of crack just after having used gateway drugs. These findings support the concept that drug abuse prevention programs, even in the face of a new drug epidemic, should be aimed at youth and not adults. Those adults who are not already hard-drug users are unlikely to begin hard-drug abuse with the newfound and growing popularity of a particular drug. 3 tables, 1 figure, and 36 references