A case-control study was performed to assess the potential influence of catechol O-methyl transferase (COMT) genotype on the risk of breast cancer in Korean women. One hundred and sixty-three histologically confirmed incident breast cancer cases and 163 age- and menopausal status-matched control individuals with no present or previous history of cancer were selected as study subjects. COMT genetic polymorphism was determined by gel electrophoresis after NlaIII enzyme digestion of amplified DNA. Odds ratios and 95% confidence intervals were estimated by unconditional logistic regression after adjustment for known or suspected risk factors of breast cancer. Women with at least one COMT lower enzyme activity associated allele (COMT-L) were at elevated risk for breast cancer (OR = 1.7, 95% CI = 1.04-2.78) compared with those homozygous for high enzyme activity associated COMT-H alleles. Among women with low (> or = 23.1) body mass index the COMT-L allele containing genotypes posed a marginally significant increased risk of breast cancer compared to the COMT-HH genotype (OR = 1.8, 95% CI = 0.95-3.48). Women with at least one COMT-L allele who had experienced a full-term pregnancy when aged over 30 years or were nulliparous had 2.7-fold increased risk; however, this increase did not reach statistical significance (OR = 2.7, 95% CI = 0.64-11.35). Furthermore, never-drinking and never-smoking women with at least one COMT-L allele were at increased risk of breast cancer compared to those with COMT-HH genotype with ORs of 2.0 (95% CI = 1.23-3.38) and 1.7 (95% CI = 1.04-2.62), respectively. These results are consistent with studies showing that COMT genotype of lower enzyme activity might be related to increase in risk of breast cancer, and extend this finding to Korean women.