Microsatellite instability (MSI) occurs in approximately 15% of colon tumors. Other than relatively rare mutations in mismatch repair genes, the causes of MSI are not generally known. The purpose of this study was to determine if dietary intake of nutrients previously reported as being associated with colon cancer relate specifically to the MSI disease pathway. Data from a population-based case-control study of adenocarcinoma of the colon were used to evaluate associations between dietary intake and MSI. Participants were between 30 and 79 years of age at time of diagnosis and included both men and women. Dietary intake data were obtained from a computerized diet history questionnaire. MSI was evaluated in several ways: by a panel of 10 tetranucleotide repeats, and by 2 mononucleotide repeats, BAT-26 and TGFbetaRII. A total of 1,510 cases had valid study data and tumor DNA on which we were able to obtain MSI status. Cases with and without MSI were compared with dietary data reported by 2,410 population-based controls to determine dietary associations that may be different for these 2 subsets of cases. We compared dietary intake for cases with and without MSI to further determine associations that are specific to the MSI disease pathway. When comparing MSI+ to MSI- tumors we observed that long-term alcohol consumption, especially intake of liquor, increased the probability of having a tumor with MSI [odds ratio (OR) for MSI+ vs. MSI- tumors for alcohol 1.6, 95% confidence interval (CI) 1.0-2.5; OR for liquor 1.6, 95% CI 1.1-2.4]. The likelihood of having MSI in the tumor from the combined effects of high alcohol consumption and smoking cigarettes showed a 70% excess in risk from the additive model. There were some suggestions that high intakes of refined grain might also be associated with MSI+ tumors, although associations were less consistent. Risk estimates for most other dietary factors did not differ substantially by MSI status. Data from this large population-based case-control study of colon cancer indicate that alcohol consumption, especially consumption of liquor, may increase the odds of an MSI+ tumor. Most other dietary factors do not appear operate exclusively in the MSI+ disease pathway.