OBJECTIVE: Although studies reveal substantial mental health treatment needs among youths in the juvenile justice system, far less is known about young offenders transferred to adult criminal court. This statewide study examined the mental health needs of young offenders who committed serious crimes and were transferred to adult court and subsequently incarcerated in a prison for adults. METHODS: Sixty-four boys aged 16 and 17 years who were incarcerated in the Texas adult correctional system completed the Massachusetts Youth Screening Instrument-Version 2 (MAYSI-2), a mental health screening measure widely used in the juvenile justice system. Scores from the youths in adult prison were compared with those of a matched sample of youths in juvenile correctional facilities, drawn from the MAYSI-2 normative data. RESULTS: Youths in adult prison reported substantial symptoms of mental health problems. Most youths surveyed (51%) scored above the highest clinical cutoff (the "warning" range) on at least one MAYSI-2 subscale. For every clinical subscale except suicide ideation, the majority of youths (54% to 70%, depending on the subscale) scored above the "caution" range. Juveniles in adult prison reported higher rates of symptoms than did those in juvenile correctional facilities (effect sizes ranged from d=.18 to d=.65, depending on the subscale). CONCLUSIONS: Although the mental health needs of youths in the juvenile justice system are well documented, this study reveals that mental health treatment needs appear to be even more pronounced in the small subgroup of youths transferred to the adult criminal justice system and incarcerated in adult prison.