54 patients have been studied after paracetamol (acetaminophen) overdose. Liver-function tests and fasting serum bile-acids were measured daily; liver biopsy was done in all cases, and slides were examined "blind" to assess liver damage. The plasma-paracetamol was measured on one occasion. A histological abnormality was present in the livers of 53 of the 54 patients, and was minor in 23, moderate in 16, and severe in 14. In 6 patients with moderate and 18 with mild histological abnormality liver-function tests were normal. A serum-aspartate-aminotransferase above 400 units/1 was always associated with severe histological liver damage. Fasting serum bile-acids were raised in 51 of the patients with abnormal liver histology; the serum-bile-acid seemed to be a more sensitive indicator of mild liver-cell damage than was the transaminase level. There was, however, little correlation between increase in bile-acid concentration and the degree of histological abnormality. As a result of these investigations empirically determined levels of plasma-paracetamol have been drawn which give a guide to the likelihood of liver damage after paracetamol overdose.