To advance clinical care and research in children with intellectual disability and autism there is a growing need for efficient means to measure behavioral severity and response to treatment. The objective of this study was to assess the feasibility of telephone administration of the Aberrant Behavior Checklist-Irritability Subscale (ABC-I). The ABC-I was administered by telephone to the primary caregivers of 39 subjects with intellectual disability and/or autism. The same primary caregiver of each subject was also mailed a written copy of the ABC-I with a self-addressed, stamped envelope. Scores obtained by telephone and written administration were highly correlated (r = 0.827, p < 0.001). Telephone administration of the ABC-I may be a feasible and efficient means of determining response to treatment in children with intellectual disability and/or autism, though these pilot findings need to be replicated in a larger sample.