Scores from the Child Behavior Checklist (CBCL; Achenbach, 1991a) and the Client Development Evaluation Report (CDER; California Department of Developmental Services, 1980) for 67 children and adolescents with mental retardation were examined to evaluate the factorial validity of the instruments. Four factor analyses were conducted. The initial factor analysis of CBCL data failed to confirm the presence of the five first-order factors previously reported for the CBCL standardization sample (Achenbach, 1991b). Second, the higher-order factors of Externalizing and Internalizing behaviors, similar to the structure reported for the CBCL standardization sample (Achenbach, 1991b), were confirmed on the present sample. Third, the two CDER factors of Personal Maladaption and Social Maladaption, previously identified by Widaman, Gibbs, and Geary (1987), were also confirmed. Finally, a higher-order factor analysis of the two factor scores from the CBCL and two factor scores from the CDER was conducted to study the congruence between the CBCL Externalizing and CDER Social Maladaption dimensions, and between the CBCL Internalizing and CDER Personal Maladaption factors. Moderate levels of congruence were found. Next, child characteristics, including level of mental retardation, age, and four dimensions of adaptive behavior, were used as predictors of problem behavior. No child characteristics were significantly related to the CBCL Externalizing dimension, but child age and level of mental retardation were significant predictors of the CBCL Internalizing dimension. CDER Cognitive Competence predicted CDER Social Maladaption, and child age predicted CDER Personal Maladaption. The findings are discussed in relation to previous studies of problem behaviors of children and adolescents with mental retardation.