This paper describes the rationale and methodology of the first National Psychiatric Morbidity Surveys to be carried out in Great Britain. The objectives of the surveys were to estimate the prevalence of psychiatric morbidity among adults aged 16-64 living in Great Britain; to identify the nature and extent of social disabilities associated with psychiatric morbidity; to describe the use of health and social services by people with psychiatric morbidity and to investigate the association between mental illness and potential environmental risk factors in a household sample. Four separate surveys were carried out in order to meet the objectives; a private household sample (n = 10,108), a sample of institutions caring for the mentally ill (n = 1191), a sample of homeless people (n = 1166), and a supplementary sample of patients with psychosis living in private households (n = 350). A two-stage assessment procedure was used, in which all subjects were given the Revised Clinical Interview Schedule (CIS-R) administered by lay interviewers to assess neurotic symptoms and disorders and a psychosis screen, including the Psychosis Screening Questionnaire. Those who were positive on the psychosis screen were then interviewed by psychiatrists using the SCAN (incorporating the tenth edition of the Present State Examination). Large scale national surveys such as this augment the inadequate data on psychiatric morbidity that are routinely available and are, therefore, an important source of information upon which to base policy and generate aetiological hypotheses. These surveys provide a possible model for similar surveys in other countries.