Existing definitions of clinical syndromes in acute liver failure do not accurately reflect important differences in clinical features and prognosis. Based on a large series of patients with acute liver failure treated at King's College Hospital, London between 1972 and 1985, we propose a new terminology. Hyperacute liver failure is our suggested term for cases in which encephalopathy occurs within 7 days of the onset of jaundice; this group includes the sizeable cohort likely to survive with medical management despite the high incidence of cerebral oedema. We suggest the term acute liver failure for cases with an interval of between 8 and 28 days from jaundice to encephalopathy; they also have a high incidence of cerebral oedema, but have a much poorer prognosis without liver transplantation. The term subacute liver failure is suggested to describe cases with encephalopathy that occurs within 5 to 12 weeks of the onset of jaundice; these patients are characterised by a low incidence of cerebral oedema, but have a poor prognosis. Adoption of this terminology should help in the management of these patients, in addition to standardising the structure and interpretation of controlled trials of therapies.