The Massachusetts Youth Screening Instrument Second Version (MAYSI-2) is a 52-item self-report measure, with content relevant to common mental health problems in delinquent populations. The measure was administered to 1,279 male and female youths, aged 12 to 17, in Massachusetts juvenile justice facilities. A subsample of youths also received the Millon Adolescent Clinical Inventory (MACI) and the Youth Self-Report (YSR). The instrument was subsequently administered to 3,804 male and female youths in California Youth Authority custody. Factor analyses of the 52 items identified seven scales that were conceptually associated with various mental, emotional, and behavioral problems of youths, which were generally confirmed by separate factor analyses with the California sample. The scales manifested adequate internal consistency and test-retest reliability. Most scales were substantially correlated with conceptually similar scales in the MACI and YSR, and they identified most youths who scored at clinically significant levels on those instruments. The article concludes that the MAYSI-2 shows promise as a reliable and valid screening tool to assist juvenile justice staff in identifying youths who may need an immediate response to mental or emotional problems. Tables, references