Previous studies have suggested that arylsulphatase A (ASA - the biochemical marker of metachromatic leucodystrophy) deficiency may be present in a sizeable proportion of patients with chronic psychosis. This study surveyed leucocyte ASA activity in a group of chronic psychotic patients and compared ASA activity in 3 subgroups fulfilling Research Diagnostic Criteria for schizophrenia (undifferentiated), paranoid schizophrenia and schizoaffective psychosis. Three of 45 patients had significantly reduced ASA activity but none had metachromatic leucodystrophy. Although ASA levels did not differ significantly between the groups, schizophrenics without a family history of schizophrenia had significantly lower ASA levels than those with. The implications of these findings are discussed.