A randomised controlled trial of the use of intravenous cysteamine in the treatment of severe paracetamol poisoning has been performed. Thirty-eight patients presenting 3-17 h after ingestion were admitted to the trial; of these eighteen received cysteamine. Two patients died from hepatic failure, one in each treatment group. Analysis of the series as a whole showed no advantage of cysteamine in preventing biochemical abnormalities of liver function except for aspartate aminotranferase and serum ferritin levels, which were significantly less after cysteamine therapy. Separate analysis of the patients treated within 9 h of paractamol ingestion and of those treated 9-17 h after paracetamol ingesion similarly showed no definite advantage of cysteamine. Histological evidence of liver damage showed a possible beneficial effect of cysteamine. Cysteamine therapy did not prevent renal or pancreatic damage.