A total of 157 youths (39.5 percent female, mean age 15.91 years) were recruited from adolescent inpatient alcohol and drug treatment centers and assessed at 6 and 12 months after discharge. Of the youths studied, 79 percent had used one or more substances by 12-month follow-up. Although only 1 percent reported alcohol as their substance of choice while hospitalized, alcohol was involved in 46 percent of initial posttreatment use episodes. Almost one-quarter (23 percent) of initial posttreatment substance use events involved multiple substances. Initial posttreatment use episodes that involved multiple substances and preferred substances were associated with poorer outcomes during the year following treatment, both in terms of rate of return to and severity of substance involvement. Youths who initially used only alcohol were also likely to resume harmful levels of substance involvement by 1 year after treatment. Results suggest two routes, "gateway" and preferred substance, for adolescents in the resumption of harmful substance use following treatment. This gateway role merits consideration by adolescent substance abuse treatment providers and families of treated youths. 3 figures, 5 tables, and 25 references