In patients without a history of diabetes mellitus, increased levels of glycated hemoglobin (HbA1c) are associated with higher cardiovascular risk. The relation between undetected diabetes and clinical outcome after percutaneous coronary intervention is unknown. To investigate whether these patients may have an increased risk of periprocedural myocardial infarction (PMI), the most frequent adverse event after percutaneous coronary intervention, we assessed patients of the TWENTE trial (a randomized, controlled, second-generation drug-eluting stent trial) in whom HbA1c data were available. Patients were classified as known diabetics or patients without a history of diabetes who were subdivided into undetected diabetics (HbA1c ≥6.5%) and nondiabetics (HbA1c <6.5%). Systematic measurement of cardiac biomarkers and electrocardiographic assessment were performed. One-year clinical outcome was also compared. Of 626 patients, 44 (7%) were undetected diabetics, 181 (29%) were known diabetics, and 401 (64%) were nondiabetics. In undetected diabetics the PMI rate was higher than in nondiabetics (13.6% vs 3.7%, p = 0.01) and known diabetics (13.6% vs 6.1%, p = 0.11). Multivariate analysis adjusting for covariates confirmed a significantly higher PMI risk in undetected diabetics compared to nondiabetics (odds ratio 6.13, 95% confidence interval 2.07 to 18.13, p = 0.001) and known diabetics (odds ratio 3.73, 95% confidence interval 1.17 to 11.89, p = 0.03). After 1 year, target vessel MI rate was significantly higher in undetected diabetics (p = 0.02) than in nondiabetics, which was related mainly to differences in PMI. Target vessel failure was numerically larger in unknown diabetics than in nondiabetics, but this difference did not reach statistical significance (13.6% vs 8.0%, p = 0.25). In conclusion, undetected diabetics were shown to have an increased risk of PMI.