Irritability is an important symptom in patients with neuropsychiatric disorders. It is a major source of distress to patients and their carers and can lead to social and family dysfunction. Despite this, there has been little systematic research on irritability in psychiatry. The development of an instrument that captures the various components of irritability is a prerequisite to more detailed research in this area. The aim of this study was to design a scale to measure irritable mood and to explore its nature and subtypes. Following a review of the literature and examination of current theories in affective neuroscience, a new self-rating questionnaire was developed covering a range of subjective experiences, judgements and behaviours deemed to encompass the components of irritability. The items were rated along intensity and frequency dimensions. The questionnaire was administered to patients with affective disorders (n=22), Huntington's disease (n=23), Alzheimer's disease (n=19) and a control group (n=46). The new questionnaire shows good reliability and validity. Preliminary differences in irritability were identified between the diagnostic groups.