This paper describes a study assessing the correlates and possible determinants of 'somatisation' by patients consulting their family doctor with a new illness. Operational criteria of 'somatisation' are given and the relationship between DSM III psychiatric disorders and physical illness are described. The epidemiological findings that are presented illustrate that 'somatisation' is a common phenomenon and accounts for a substantial proportion of 'hidden psychiatric morbidity' in primary care.