High-risk offenders treated by California’s Proposition 36 court-supervised drug treatment initiative account for a disproportionate number of re-arrests (Hawken 2008) undermining the many successes of the program, yet little is known about their characteristics, treatment experiences, or factors that influence re-arrest. To better understand this group, self-reported and administrative data were analyzed on 78 high-risk (five or more convictions in the previous 5 years) and 1,009 low-risk offenders enrolled during 2004. At intake, high-risk offenders were younger, more were male, and more had prior contact with psychiatric and criminal justice systems. Treatment received and the proportion recidivated during the 30-months after treatment assessment were similar across groups, but high-risk offenders had a greater number of re-arrests. The number of re-arrests was increased by high-risk classification, but decreased by receipt of more treatment services and longer treatment length. Moreover, the number of re-arrests was highest among high-risk offenders with shorter treatment lengths, whereas it was similar to that among low-risk offenders if treatment length was longer. To reduce recidivism among high-risk offenders in court-supervised drug treatment, consideration of psychiatric problems and criminal history is needed, as is receipt of sufficient treatment.