AbstractConsumption of red meat has been associated with elevated risk of colorectal cancer; however, mechanisms underlying this relationship are not well established. N‐acetyltransferase 2 (NAT2) appears to activate carcinogenic heterocyclic amines found in meat as well as cigarette smoke. Genetic variation in this enzyme, associated with rapid acetylation, may modulate the influence of meat intake on cancer risk. We examined the risk of incident colorectal cancer according to NAT2 genotypes, meat intake and smoking in a prospective, nested case‐control study among 32,826 women enrolled in the Nurses' Health Study who provided prediagnostic blood specimens. We matched 183 women with colorectal cancer to 443 controls. Although acetylator genotype alone was not associated with the risk of colorectal cancer, women with rapid acetylator genotypes experienced a greater risk associated with intake of ≥0.5 serving of beef, pork or lamb as a main dish per day compared to intake of less meat (multivariate OR = 3.01; 95% CI = 1.10–8.18). In contrast, among slow acetylators, intake of beef, pork or lamb was not associated with risk of colorectal cancer (multivariate OR = 0.87; 95% CI = 0.35–2.17). The interaction between acetylator genotype and meat intake approached statistical significance (Pinteraction = 0.07). Moreover, compared to slow acetylators who smoked ≤35 pack‐years and ate <0.5 serving/day of red meat, the OR for rapid acetylators who smoked >35 pack‐years and ate ≥0.5 serving/day was 17.6 (95% CI 2.0–158.3). These prospective data suggest that red meat may increase the risk of colorectal cancer, particularly among genetically susceptible individuals. The influence of NAT2 genotype on this association supports a role for heterocyclic amines in mediating the effect of red meat on colorectal carcinogenesis. © 2005 Wiley‐Liss, Inc.