Although rates of substance abuse continue to rise among young adults ages 18-25, there is little information on the clinical characteristics of young adults referred to treatment, their readiness to change substance use behaviors, and treatment outcomes. These aspects were examined in young (18-25 years old) and older adult (26-45 years old) substance abusers entering treatment at a large, urban, outpatient substance abuse treatment facility. All clients were referred for outpatient treatment by the local Offices of Adult Probation during 1998-1999. The demographic and substance use characteristics, motivation level/readiness to change substance use behaviors, treatment attendance, completion, and "drug-free" status based on patient self-report and urine/breathalyzer data were assessed. Results. Substance-abusing young adult probationers were more likely to be African-American, with a significantly earlier age of onset of primary substance use. They were more likely to have a marijuana use disorder as compared with older adults, who in turn, were more likely to meet criteria for alcohol use disorders. Furthermore, as compared with older adults, young adults had significantly higher scores on precontemplation, and significantly lower scores on contemplation, determination, action, and maintenance subscales of motivation/change readiness. A significantly higher number of young adults did not complete outpatient treatment and were "drug-positive" at discharge, as compared with older adults. Conclusions. Young adult probationers referred to substance abuse treatment show significantly different substance abuse and treatment characteristics as compared with their older adult counterparts. The findings suggest that specialized treatment approaches that focus on enhancing treatment readiness and motivation to change substance use behaviors may be of particular benefit to substance abusing young adults.