Background: The Adult Scale of Hostility and Aggression (A-SHARP) rating scale assesses the frequency/severity (problem scale) and the reactive-proactive motivation (provocation scale) of aggressive behaviors in adults with intellectual disabilities (ID). Items are assigned to five subscales (Verbal Aggression, Physical Aggression, Hostile Affect, Covert Aggression, and Bullying). Although psychometric properties reported by the scale's developers were very good, we wanted to corroborate them independently. We were also interested in whether the reactive-proactive distinction of aggressive behavior is related to a behavioral/functional classification. Method: Staff at a day-treatment program for adults with ID completed ratings for 155 clients using the A-SHARP, the Behavior Problems Inventory-01 (BPI-01), and the Questions about Behavioral Function (QABF). Results: Internal consistency was found to be excellent, and the A-SHARP Physical Aggression subscale had good congruent and clinical validity. Confirmatory factor analysis showed sufficient evidence toward the factorial validity of the A-SHARP's problem scale. The reactive-proactive classification of aggressive behavior motivation by the A-SHARP's provocation scale was independent of the functional classification. Conclusions: The A-SHARP is a useful addition to a small number of existing instruments for assessing aggressive behavior in adults with ID, especially its problem scale. We discuss how the A-SHARP provocation scales might provide therapy-relevant information. Empirical evidence for the clinical utility of the A-SHARP provocation scale will have to be established by future research.