BACKGROUND AND AIMS: Variant genotypes of uridine diphosphate glucuronsyltransferase isoenzyme 1A6 (UGT1A6) associated with decreased metabolic activity have been associated with an enhanced protective effect of aspirin on the development of colorectal adenomas. However, interactions between UGT1A6 variants or variants of another enzyme that metabolizes nonsteroidal anti-inflammatory drugs (NSAIDs), cytochrome P4502C9 (CYP2C9), and NSAIDs in the prevention of colorectal cancer have not been studied extensively. METHODS: UGT1A6 and CYP2C9 genotypes were determined in 2295 individuals with colorectal cancer and 2903 controls. Interactions between these genotypes, aspirin or ibuprofen use, and colorectal cancer risk were determined. RESULTS: Variant CYP2C9 genotypes enhanced the protective effect of ibuprofen on the prevention of colorectal cancer, and a dose-response relationship with respect to increasing numbers of variant alleles was seen (P interaction = .02). CYP2C9 variants were more effective in individuals with wild-type rather than variant UGT1A6 (P interaction < .007). Variant CYP2C9 genotypes showed no interaction with aspirin usage, and variant UGT1A6 genotypes showed no interaction with either NSAID with respect to colorectal cancer protection. CONCLUSIONS: In this study, the major effect seen was an enhancement by slower-metabolizing CYP2C9 variants of the chemopreventive activity of ibuprofen against colorectal cancer.