Background: Diabetes mellitus is reported to increase the risk of colorectal cancer (CRC) development and has been associated with poor tumour-specific outcomes. Here we assessed the impact of diabetes on the clinicopathologic features and tumour mutation profiles of CRC. Methods: Analysis of a prospective series of patients diagnosed with CRC between January 2000 and December 2010. Fresh-frozen and formalin-fixed, paraffin-embedded tumour specimens were retrieved and genomic DNA extracted for analysis of microsatellite instability (MSI), CpG island methylator phenotype (CIMP) and mutations in BRAF, KRAS, PIK3CA, TP53 and APC genes. Propensity-score matching and logistic regression were used to estimate the association of diabetes with tumour molecular profile, controlling for age, sex, tumour stage, body mass index (BMI), smoking and socio-economic status. Results: Of the 1,348 patients assessed, 288 (21.4%) had a history of diabetes mellitus. Compared to patients without diabetes, diabetics were more likely to be older (age > 70 yrs: 56% vs 47%, p = 0.006), male (57% vs 47%, p = 0.002) and have a higher BMI (BMI > 25: 82% vs 65%, p < 0.0001). There were no statistically significant differences in tumour site, differentiation or lymphovascular invasion. Propensity scores were used to match 260 diabetic patients to an equal number of nondiabetics. In multivariate regression analysis, diabetes was associated with BRAF-mutated tumours (OR 2.81, 95% CI 1.16-7.59, p = 0.029) and showed a trend towards MSI-high tumours (OR 1.54, 95% CI 0.91-2.63, p = 0.110). Results are shown in the Table. Survival analysis is planned. Conclusions: CRC patients with diabetes are older, more likely male and have higher BMI than non-diabetics. In this preliminary analysis, an association between diabetes and BRAF-mutant CRC was found, and may explain reported differences in outcomes for diabetic CRC patients.