Heterocyclic amines found in well-done meat require host-mediated metabolic activation before initiating DNA mutations and tumors in target organs. Polymorphic N-acetyltransferase-2 (NAT2) catalyzes the activation of heterocyclic amines via O-acetylation, suggesting that NAT2 genotypes with high O-acetyltransferase activity (rapid/intermediate acetylator phenotype) increase the risk of breast cancer in women who consume well-done meat. To test this hypothesis, DNA samples and information on diet and other breast cancer risk factors were obtained from a nested case-control study of postmenopausal women. Twenty-seven NAT2 genotypes were determined and assigned to rapid, intermediate, or slow acetylator groups based on published characterizations of recombinant NAT2 allozymes. NAT2 genotype alone was not associated with breast cancer risk. A significant dose-response relationship was observed between breast cancer risk and consumption of well-done meat among women with the rapid/intermediate NAT2 genotype (trend test, P = 0.003) that was not evident among women with the slow acetylator genotype (trend test, P = 0.22). These results suggest an interaction between NAT2 genotype and meat doneness, although a test for multiplicative interaction was not statistically significant (P = 0.06). Among women with the rapid/intermediate NAT2 genotype, consumption of well-done meat was associated with a nearly 8-fold (odds ratio, 7.6; 95% confidence interval, 1.1-50.4) elevated breast cancer risk compared with those consuming rare or medium-done meats. These results are consistent with a role for O-acetylation in the activation of heterocyclic amine carcinogens and support the hypothesis that the NAT2 acetylation polymorphism is a breast cancer risk factor among postmenopausal women with high levels of heterocyclic amine exposure.