Anger and associated aggressive behaviour are significant clinical issues for many people with intellectual disabilities (IDs) that can lead to serious constraints to their liberty, which, in turn, adversely affects their quality of life. There is some evidence to support cognitive-behavioural anger treatment in this client group; however, anger assessment protocols for people with IDs should be diversified. In this regard, a method for anger assessment using imaginal provocation scenes was extended for use with this client population and the context in which treatment takes place. Two studies of the Imaginal Provocation Test (IPT) were conducted: the first with 48 patients examined its internal reliability and concurrent validity with anger psychometric scales; the second investigated whether it was sensitive to change associated with anger treatment in a small outcome study involving men with IDs and histories of offending. The IPT was found to successfully induce anger, be internally reliable, have strong concurrent validity and detect statistically significant changes in anger following anger treatment (N = 9), compared with a routine care waiting-list control group (N = 8). The IPT also had value in highlighting clinical improvements for anger treatment condition participants compared with the control group.