Adolescents who received services in one or more of the following public sectors of care were included: alcohol and drug (AD); juvenile justice (JJ); mental health (MH); public school-based services for youth with serious emotional disturbance (SED); and child welfare (CW), in relation to age, gender, and service sector affiliation. The participants in this study included 1,036 adolescents aged 13- to 18-years-old, randomly sampled from all youths who were active in at least one of the above five sectors of care in San Diego County, California. SUDs were assessed through structured diagnostic interviews conducted from October 1997 through January 1999. The results showed that SUDs were found for youths in all sectors of care, with lifetime rate of 82.6 percent in AD, 62.1 percent in JJ, 40.8 percent in MH, 23.6 percent in SED, and 19.2 percent in CW. The rates of SUDs were significantly higher among older youths and males. Sector differences held even when accounting for the effects of age and gender. Clinicians and other public service sector professionals should be sensitive to the likelihood of SUDs in settings not specifically providing substance abuse treatment. It is likely that the use of specific substances may be related to the need for and the type, intensity, or duration of services. 1 figure, 2 tables, 50 references