SummaryBipolar and unipolar patients respectively were separated into psychotic and non-psychotic sub-types. The bipolar psychotic patients were more likely to have certain severe symptoms, such as hallucinations and motor abnormalities, than were the unipolar patients, but the family histories of the four sub-groups were identical. The psychotic sub-groups had a different course of illness, in that they were less likely to have had a history of multiple episodes on admission and were more likely to show chronicity for a period of time on discharge. The data are interpreted as being opposed to the concept of a continuum of vulnerability in the affective disorders, and as not favouring either psychotic unipolar or psychotic bipolar illnesses or schizo-affective disorder being considered autonomous. One possible interpretation of the findings is that a trait or propensity to psychosis is transmitted totally independently of the major affective illness, and that this propensity is silent or not observed when the patient is in remission.