Patients with mental illness are exposed and experience themselves as not being taken seriously in emergency care. Registered nurses need to assess patients with mental illness from a holistic perspective comprising both a physical and an existential dimension. The aim of the study was to describe registered nurses’ (RNs) experiences of assessing patients with mental illness in emergency care. Twenty-eight RNs in prehospital and in-hospital emergency care were individually interviewed. The interviews were analysed descriptively. The design followed the COREQ-checklist. One main theme ‘A conditional patient assessment’ and two themes; ‘A challenged professional role’ and ‘A limited openness for the patient’, comprising in turn four sub-themes emerged. Although the RNs showed willingness to understand the mental illness aspects of their patients, they were insufficient in their assessments. This implies the importance of developing emergency care RNs’ competence, knowledge and self-confidence in assessments and care of patients with mental illness.