O6-methylguanine DNA methyltransferase (MGMT) is a repair protein that transfers methyl groups from O6-methylguanine to a cysteine acceptor in its own molecule, and restores DNA to its undamaged state. If left unrepaired, O6-methylguanine can pair with either a thymine or a cytosine, causing a C-G to T-A transition, which is considered to be one of the molecular mechanisms of both mutagenesis and carcinogenesis. The expression of MGMT mRNA in liver tissue was quantitatively assessed by the competitive reverse transcription-polymerase chain reaction method in patients with chronic liver diseases with or without alcohol drinking. MGMT mRNA expression was 1.4 +/- 0.9 pg/micrograms RNA in control livers. Its expression in chronic hepatitis was 3.8 +/- 0.7 in alcoholics and 2.7 +/- 0.8 in nonalcoholics, which were not statistically different. MGMT mRNA expression in liver cirrhosis was significantly low, compared with that in chronic hepatitis, and 0.8 +/- 0.3 in alcoholics and 0.5 +/- 0.1 in nonalcoholics, which also were not significantly different. The present study shows that MGMT mRNA was not decreased in patients with chronic liver diseases with alcohol drinking, compared with those without alcohol drinking.