BACKGROUND CYP2E1, the coding gene for the ethanol inducible cytochrome P4502E1, is polymorphic at the RsaI restriction site in the 5' flanking region. The mutant allele c2 has a higher transcriptional activity than the wild-type gene c1. P4502E1 catalyses the activation of several environmental carcinogens at a rate that is increased, if only moderately, by longterm ethanol intake. OBJECTIVE To establish the distribution of CYP2E1 RsaI polymorphism in patients with hepatocellular carcinoma and to evaluate its possible role in the multifactorial pathogenesis of this tumour. METHODS 101 (84 males) patients with hepatocellular carcinoma and 178 (128 males) healthy controls of the same ethnic (white) and Spanish origin. METHODS After extraction of DNA from white blood cells, alleles c1 and c2 of CYP2E1 were identified by restriction fragment length polymorphism (RFLP) with endonuclease RsaI. RESULTS Homozygous c1c1: 90 patients and 169 controls; heterozygous c1c2: 11 and 9; homozygous c2c2: none (non-significant difference). C2 allele frequencies: 0.055 in patients, 0.025 in controls (non-significant difference) and 0.108 in the 37 patients who had drunk more than 50 g of ethanol/day (p = 0.0035, odds ratio versus controls: 4.67; 95% confidence limits 1.57 to 13.81). CONCLUSIONS The carrier state of one copy of the c2 CYP2E1 gene increases the risk of hepatoma in previously regular ethanol users with chronic liver disease.