The aim of this study was to investigate whether polymorphisms in N-acetyl transferase 1 and 2 modify the association between meat consumption and risk of breast cancer. A nested case-control study was conducted among 24 697 postmenopausal women included in the 'Diet, Cancer and Health' cohort study (1993-2000). Three hundred and seventy-eight breast cancer cases were identified and matched to 378 controls. The incidence rate ratio (95% confidence interval) for breast cancer was 1.09 (1.02-1.17) for total meat, 1.15 (1.01-1.31) for red meat and 1.23 (1.04-1.45) for processed meat per 25 g daily increment in intake. Compared with slow acetylators, the IRR (95% confidence interval) among fast N-acetyl transferase 1 acetylators was 1.43 (1.03-1.99) and 1.13 (0.83-1.54) among intermediate/fast N-acetyl transferase 2 acetylators. Interaction analyses revealed that the positive associations between total meat intake and red meat intake and breast cancer risk were confined to intermediate/fast N-acetyl transferase 2 acetylators (Pinteraction=0.03 and 0.04). Our findings support an association between meat consumption and breast cancer risk and that N-acetyl transferase 2 polymorphism has a modifying effect on the association, indicating that the association is confined to only genetically susceptible women.