The Aberrant Behavior Checklist (ABC; Aman, Singh, Stewart, & Field, 1985a, 1985b) is a 58-item third-party informant rating scale originally developed for institutionalized, low-functioning adolescents and adults. The present study investigated the appropriateness of the scale for youngsters with dual diagnosis of mental retardation and psychiatric disturbance. Over a period of 2 1/2 years, 204 patients (199 after data reduction) from a child psychiatry unit were rated twice daily by direct care staff. Data analysis addressed internal consistency, interrater reliability, criterion validity, and robustness of the factor structure. Internal consistency was satisfactory with alpha coefficients ranging from .82 to .94. Interrater reliability varied between subscales but was relatively low (Pearson correlations between .39 to .61). In terms of its criterion validity, the ABC was sensitive to psychiatric diagnoses and age and the original 5-factor structure was robust (congruence coefficients ranged between .80 to .89). Yet, only a relatively small proportion of the variance (31.5%) was explained by factor analysis indicating possible limitations of the ABC for this population. Given the paucity of assessment instruments for this particular population and the difficulty involved in developing new population-specific instruments, the ABC can be recommended for children and adolescents with dual diagnosis.