Millon Adolescent Personality Inventory differential diagnosis of disruptive behavior disorder vs no behavior disorder was investigated. Results were twice cross-validated. Millon adolescent Scales 4, 5, 6, G, SS, and TT were closely related to diagnosis of behavior disorder. Two psychometric rules averaged 70% hits and 35% false positives over three samples, Rule 1 of > 2 elevations (baserate score > 63) among Scales 6-G-SS-TT and Rule 2 of Scale 4 elevated with Scale 5 or with one of 6-G-SS-TT. An ancillary rule was highly specific to behavior disorder (90%). Two broad dimensions or factors appeared to be represented in the key scales; Scales 4 and 5 exemplified a histrionic-narcissistic factor, while the 6-G-SS-TT combination typified behavioral conflict and instability.