Behavior problems such as aggression, property destruction, stereotypy, self-injurious behavior, and other disruptive behavior are commonly observed among adults with intellectual disabilities (ID), autism spectrum disorders (ASD), and epilepsy residing at state-run facilities. However, it is unknown how these populations differ on behavior problem indicies. Assessment of behavior problems were made with the ASD–behavior problems-adult version battery. One hundred participants with ID were matched and compared across four equal groups comprising 25 participants with ID, 25 participants with epilepsy, 25 participants with ASD, and 25 participants with combined ASD and epilepsy. When controlling for age, gender, race, level of ID, and hearing and visual impairments, significant differences were found among the four groups, Wilks's Λ =.79, F(12, 246)=1.93, p <.05. The multivariate η 2 based on Wilks's Λ was .08. A one-way ANOVA was conducted for each of the four subscales of the ASD–BPA as follow-up tests to the MANOVA. Groups differed on the aggression/destruction subscale, F(3, 96)=.79, p >.05, η 2 =.03, and stereotypy subscale, F(3, 96)=2.62, p >.05, η 2 =.08. No significant differences were found on the self-injury subscale and disruptive behavior subscale. Trend analysis demonstrated that individuals with ID expressing combined co-morbid ASD and epilepsy were significantly more impaired than the control group (ID only) or groups containing only a single co-morbid factor with ID (ASD or epilepsy only) on these four subscales. Implications of these findings in the context of known issues in ID, epilepsy, and ASD, current assessment practices among these populations and associated challenges are discussed.