Microsomal epoxide hydrolase (mEH) metabolizes polycyclic aromatic hydrocarbons, carcinogens found in cigarette smoke and cooked meat. Polymorphisms in exon 3 and exon 4 of the mEH gene have been found to alter mEH activity. We investigated the association between these polymorphisms and colorectal polyps within the Minnesota Cancer Prevention Research Unit case-control study. Cases were diagnosed with colonoscopically confirmed adenomas (n = 530) or hyperplastic polyps (n = 202); controls (n = 649) were polyp-free at colonoscopy. Smoking history and meat consumption were obtained from self-administered questionnaires before colonoscopy. mEH genotypes were determined by PCR/RFLP or oligonucleotide ligation assay. The overall risks associated with exon 3 or exon 4 polymorphisms for both adenomas and hyperplastic polyps were not statistically different from 1.0. Compared with exon 3 Tyr/Tyr, 0 pack-years, risk was highest among those with the exon 3 His/His genotype and >25 pack-years of smoking [adenoma, odds ratio (OR) = 4.9 (1.9-12.8); hyperplastic, OR = 7.7 (2.5-24.0)]. Risks were not elevated among exon 4 homozygous variants, even in the presence of heavy smoking. Fried, baked, or broiled meat intake of > or =two servings/week (high) compared with < or =one serving/week was associated with a 2-fold increase in risk of adenoma. The highest risks were seen for those with the exon 3 His/His genotype and high cooked meat intake [OR = 3.3 (1.4-7.9); reference group: Tyr/Tyr, < or = 1 serving/week). Although mEH polymorphisms are not associated with an increased risk of colorectal polyps overall, genotypes that produce a slow phenotype appear to be associated with an increased risk in the presence of smoking and high intakes of cooked meat.