BACKGROUND Persistent infection with hepatitis B virus (HBV) causes chronic phasic necroinflammation and regenerative proliferation in the liver. The sustained hepatocellular proliferation may render chronic HBV carriers more susceptible to the effects of environmental carcinogens. Aromatic amines are potential hepatocarcinogens in humans.N-acetyltransferase (NAT) is involved in the metabolic activation and detoxification of these compounds. AIMS To investigate if genetic polymorphisms in N-acetylation are related to hepatocellular carcinoma (HCC) among chronic HBV carriers. METHODS Genotyping ofNAT1 and NAT2 was performed using polymerase chain reaction-restriction fragment length polymorphism on peripheral leucocyte DNA from 151 incident cases of HCC and 211 controls. All subjects were male, and were chronic HBV surface antigen carriers. RESULTS A significant association between NAT2 genetic polymorphism and HCC was observed among chronic HBV carriers who were smokers but not among those who were non-smokers. For smoking HBV carriers, the odds ratios of developing HCC for those heterozygous and homozygous for the NAT2*4 functional allele compared with those without any copies of the functional allele (reference group) were 2.67 (95% confidence interval 1.15–6.22) and 2.58 (95% confidence interval 1.04–6.43), respectively. The interaction between cigarette smoking and the presence of theNAT2*4 allele just failed to reach statistical significance (p=0.06). No association betweenNAT1 genotype and HCC was evident overall or within the smoking stratified subgroups. CONCLUSIONS Our results suggest that NAT2 activity may be particularly critical in smoking related hepatocarcinogenesis among chronic HBV carriers. Our data also indirectly support a role for tobacco smoke derived aromatic amines in the aetiology of HCC.