Severe problem behaviors such as self-injury and aggression are frequently observed in young children under age 5 with intellectual and developmental disabilities (IDD). Although early identification of problem behavior is critical to effective intervention, there are few standardized measures available that identify severe problem behavior in this population. The Aberrant Behavior Checklist-Community (ABC-C; Aman & Singh, 1994) is a rating scale that measures the severity of a range of problem behaviors commonly observed in individuals with IDD. While it has been used with children under 5, investigations into the fit of the ABC-C for this population are sparse. The purpose of the present study was to report on ABC-C scores in a sample of 97 children under age 5 with problem behavior. Analyses included evaluating differences in scores between age groups, comparing sample norms to established norms for older children, and conducting a confirmatory factor analysis. Results indicated differences in mean scores based on age with younger children generally scoring higher on some subscales of the ABC-C. Furthermore, the original 5-factor structure of the ABC-C was not fully supported. In general, the ABC-C may over- or underestimate behavior problems in younger children; therefore more extensive investigation into the utility of the ABC-C for children under age 5 is warranted. (Contains 4 tables.)