OBJECTIVE--To find out whether changes in the daily prothrombin time are of prognostic importance in patients with paracetamol induced fulminant hepatic failure. DESIGN--Retrospective study. SETTING--The Liver Unit, King's College Hospital, London. PATIENTS--150 Consecutive patients with paracetamol induced fulminant hepatic failure admitted between October 1986 and February 1989. MAIN OUTCOME MEASURE--Death. RESULTS--Of the 150 patients, 72 (48%) died. In all, 34 of the 37 (92%) patients with a peak prothrombin time of greater than or equal to 180 seconds died as did 20 of the 41 (49%) with a time of 130-179 seconds, nine of the 25 (36%) with a time of 90-129 seconds, and nine of the 47 (19%) with a time of less than 90 seconds. Of the 42 patients with a continuing rise in prothrombin time between days 3 and 4 after overdose, 39 died (93%) compared with 21 of the 96 (22%) in whom the prothrombin time fell. CONCLUSIONS--These data indicate that a continued increase in prothrombin time on day 4 after overdose and a peak prothrombin time of greater than or equal to 180 seconds identify at an early stage those patients with a less than 8% chance of survival. Liver transplantation should be considered in patients meeting either of these criteria.