BACKGROUND: The Social Functioning Questionnaire (SFQ), an eight-item self-report scale (score range 0-24), was developed from the Social Functioning Schedule (SFS), a semi-structured interview which has been used primarily with non-psychotic patients and has good test-retest and inter-rater reliability as well as construct validity. The SFQ was developed following the need for a quick assessment of perceived social function. AIMS: To give further details of old and new data sets from studies involving over 4000 subjects assessed with the SFQ illustrating its epidemiological and clinical associations. METHOD: New data were analysed from a national epidemiological study, a comparison of key-worker and subject versions of the SFQ, and reanalysis of data from three earlier clinical studies, of psychiatric emergencies, general practice psychiatric patients and those with recurrent psychotic illnesses. These data were examined further to determine their range, their relationship to other clinical measures, and change over time in clinical trials. RESULTS: The population mean score in 4164 subjects was 4.6 and the data from all studies suggested that a score of 10 or more indicated poor social functioning. Those presenting as psychiatric emergencies had the poorest social function (mean 11.4) and psychiatric patients from general practice the best function (mean 7.7) of the clinical populations. The eight item scores had a normal distribution in psychiatric populations and a skewed one in a normal population; scores were relatively stable over the short (weeks) and long-term (months), and were high in the presence of acute mental health disturbance and personality disorder, giving support to the validity of the scale. The results from a UK sample of a randomly selected population specifically weighted for ethnic minorities showed similar social function across groups.