Within the scope of a double-blind, placebo-controlled, crossover medication study, the Diagnostic Assessment of the Severely Handicapped-II (DASH-II) was evaluated as a measurement for determining the effectiveness of the medication risperidone in treating the problem behaviour of 21 people with intellectual disabilities (ID). Participants caregivers completed the DASH-II during the placebobaseline phase of the study and the maintenance phase of the study, and completed the Aberrant Behavior Checklist Community (ABC-C) weekly throughout the entire study. The results obtained using the DASH-II were compared to those obtained using the ABC-C, an instrument shown to be well correlated with the DASH-II. Results suggest that while the DASH-II and the ABC-C were well correlated during the placebo/baseline phase of the current study, they were not well correlated at completion of the 6-month maintenance phase of the medication trial. The DASH-II, while appropriate for assisting in the diagnosis of psychopathology in people with ID, does not appear to monitor changes in problem behaviour as a result of risperidone use as well as the ABC-C. Differences in the frequency of problem behaviour that each measure evaluated and the applicability of using the DASH-II to measure medication effects on problem behaviour are discussed.