The reduced risk of colorectal cancer associated with cyclooxygenase enzyme inhibitors, such as aspirin and other nonsteroidal anti-inflammatory drugs, strongly suggests that chronic inflammation is a key mediator in the development of colorectal cancer. This complements recent molecular evidence demonstrating an association between a number of proinflammatory genetic polymorphisms and risk of colorectal cancer. We assessed polymorphisms in the IL-1, IL-10, TNF-A, and TGF-B genes in a population-based case-control study of colorectal cancer cases (n = 264) and frequency-matched controls (n = 408) in the Northeast of Scotland and analyzed their interaction with regular aspirin use. There was no evidence of a relation between any of the individual polymorphisms, or pairs of polymorphisms, and risk of colorectal cancer. There was a significant interaction between the IL-10-592 C/A polymorphism and aspirin use (Pinteraction = 0.03). Carriers of the variant IL-10-592 (A) allele, who produce less of the anti-inflammatory cytokine interleukin-10, had a statistically significant 50% reduced risk of colorectal cancer when taking regular aspirin (odds ratio, 0.5; 95% confidence interval, 0.25-0.97), whereas risk was not reduced in carriers of the A allele who did not use aspirin, or among aspirin users with the CC genotype. It is possible that carriers of the mutant IL-10-592 allele are more likely to derive anti-inflammatory and chemopreventive benefits from aspirin in the presence of a lower production of their own endogenous anti-inflammatory interleukin-10. These results suggest that host genetics may play a role in predicting response to chemopreventive strategies. Confirmation of these findings in other populations is required.