Background Many people with intellectual disabilities display high levels of anger, and cognitive‐behavioural anger management interventions are used routinely. However, for these methods to be used optimally, a better understanding is needed of different forms of anger assessment. The aim of this study was to investigate the relationship of a range of measures to self‐ and carer reports of anger expression, including instruments used to assess mental health and challenging behaviour. Method Adults with intellectual disabilities, who had been identified as having problems with anger control, their key‐workers and home carers all rated the service users’ trait anger, using parallel versions of the same instrument (the Provocation Inventory). In addition, service users completed a battery of mental health assessments (the Glasgow Depression Scale, Glasgow Anxiety Scale and Rosenberg Self‐Esteem Scale), and both groups of carers completed a battery of challenging behaviour measures (the Hyperactivity and Irritability domains of the Aberrant Behavior Checklist and the Modified Overt Anger Scale). Results Participants had high levels of mental health problems (depression: 34%; anxiety: 73%) and severe challenging behaviour (26%). Hierarchical linear regression analysis was used to explore the extent to which anger ratings by the three groups of respondents were predicted by demographic factors, mental health measures and challenging behaviour measures. Older service users rated themselves as less angry and were also rated as less angry by home carers, but not by key‐workers. More intellectually able service users were rated as more angry by both sets of carers, but not by the service users themselves. Significantly, mental health status (but not challenging behaviour) predicted service users’ self‐ratings of anger, whereas challenging behaviour (but not mental health status) predicted carers’ ratings of service users’ anger. Conclusions Service users and their carers appear to use different information when rating the service users’ anger. Service users’ self‐ratings reflect their internal emotional state and mental health, as reflected by their ratings of anxiety and depression, whereas staff rate service users’ anger on the basis of overt behaviours, as measured by challenging behaviour scales.