Objective: To estimate the 6-month prevalence of multiple substance use disorders (SUDs) among juvenile detainees by demographic subgroups (sex, race/ethnicity, age). Method: Participants were a randomly selected sample of 1,829 African American, non-Hispanic white, and Hispanic detainees (1,172 males, 657 females, aged 10 to 18). Patterns and prevalence of DSM-III-R multiple SUDs were assessed using the Diagnostic Interview Schedule for Children Version 2.3. The authors used two-tailed F and t tests with an [alpha] of .05 to examine combinations of SUDs by sex, race/ethnicity, and age. Results: Nearly half of the detainees had one or more SUDs; more than 21% had two or more SUDs. The most prevalent combination of SUDs was alcohol and marijuana use disorders (17.25% females, 19.42% males). Among detainees with any SUD, almost half had multiple SUDs. Among detainees with alcohol use disorder, more than 80% also had one or more drug use disorders. Among detainees with a drug use disorder, approximately 50% also had an alcohol use disorder. Conclusions: Among detained youths with any SUD, multiple SUDs are the rule, not the exception. Substance abuse treatments need to target detainees with multiple SUDs who, upon release, return to communities where services are often unavailable. Clinicians can help ensure continuity of care by working with juvenile courts and detention centers. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(10):1215-1224. Key Words: juvenile detainees, substance use disorders.