Three parent-completed behavior rating scales, the Conners Abbreviated Rating Scale (ARS), Child Behavior Checklist (CBCL), and Self-Control Rating Scale (SCRS), were studied in boys with attention deficit disorder (ADD), psychiatric controls, and a normal comparison group. The ability of the scales to discriminate these groups was determined via univariate analyses of variance (ANOVA) and discriminant function analyses. All three rating scales revealed group differences. In discriminant function analyses, the ARS was the single best predictor of group membership, but it frequently misclassified psychiatric controls. Externalizing behavior indexes from the SCRS and CBCL yielded similar results. The addition of CBCL internalizing and social competence scales improved overall group classification, facilitating the identification of psychiatric controls. Results encourage the use of multidimensional rating scales such as the CBCL in clinical differential diagnosis, but suggest that unidimensional scales such as the ARS are also useful for assessing change or treatment effects in ADD.