Little is known of how a comprehensive mental health nursing assessment is undertaken in practice. Nurses describe the content of a comprehensive mental health nursing assessment in different ways. Different content may lead to different interventions. More work is required to ensure comprehensive mental health nursing assessments are undertaken in practice. Assessment is the foundation of mental health nursing practice, but little is known of how it is undertaken. This paper explores how mental health nurses describe the content of a comprehensive mental health nursing assessment. Eighteen nurses who worked in inpatient and community settings either as clinicians or managers, ranging from new graduates to nurses with greater than 20 years of experience, were interviewed and asked to describe the content of a comprehensive mental health nursing assessment. Transcribed interviews were analysed using a grounded theory methodology. The primary theme to emerge was one of variability. Most respondents hesitated and then identified different content areas that needed to be assessed as part of a comprehensive mental health nursing assessment. If the areas that are being assessed vary between nurses, then logically the types of interventions being offered will also vary. These results have implications for the education of nurses, their clinical practice, ongoing supervision and research into contemporary mental health nursing practice.