BACKGROUND. Using improved methods, findings of higher criminality in a psychiatric population were tested. METHOD. An unselected sample of 1265 Swiss in-patients and a matched control group drawn from the general population were studied. Detailed accounts of conviction records served as a measure of criminal behaviour. RESULTS. Apart from traffic law violations by men, and violent crimes and sexual offences by women, patients were more frequently registered in all crime categories. However, there were significant differences between the diagnostic groups. Alcoholics and drug users of both sexes had a significantly higher criminality rate. A higher rate was also found among female, but not male, patients suffering from schizophrenia or related disorders and other, mostly organic, disorders. CONCLUSIONS. Alcoholism and drug abuse contribute significantly to criminal behaviour, independent of sociodemographic factors; however, with a few exceptions, mental disorders such as schizophrenia and affective disorders do not contribute to criminal behaviour.