Background: This study examines the diagnostic accuracy of the CBCL syndrome AS scales for predicting DSM-IV Attention Deficit-Hyperactivity Disorder (ADHD) and Oppositional Defiant Disorder with or without Conduct Disorder (ODD/CD). Methods: The sample included 370 children (187 probands and 183 siblings) participating in a family genetic study of attention and aggressive behavior problems. Univariate and stepwise logistic regression analyses were used to derive models for predicting two diagnostic conditions: ADHD and ODD/CD. Results: The Attention Problems syndrome significantly predicted ADHD, and ODD/CD was significantly predicted by the Aggressive Behavior syndrome. Both scales demonstrated good diagnostic accuracy, as assessed through receiver operating characteristics analyses. Cut-point analyses confirmed the utility of low T-scores, 55 on the respective syndromes, for efficiently discriminating cases from noncases. Conclusions: CBCL syndromes display good diagnostic efficiency for assessing common externalizing disorders in children.