- Background We sought to determine the prevalence of diabetes mellitus (DM) newly diagnosed by elevated glycated hemoglobin (HbA1c) in patients undergoing percutaneous coronary intervention (PCI) and its association with 1-year clinical outcomes. Methods We prospectively enrolled consecutive patients undergoing PCI (2011–2013). HbA1c levels were assessed during the index hospitalization and newly diagnosed DM was defined as HbA1c ≥ 6.5% in the absence of the previous diagnosis. The primary outcome was MACCE (Major Adverse Cerebro- and Cardiovascular Events) defined as death, stroke, PCI or acute myocardial infarction at 1 year. Results Diabetes was previously diagnosed in 391 (34%) patients (DM group), 221 (19%) had newly diagnosed DM based on the HbA1c level and 539 (47%) did not have diabetes (Non-DM). In DM group HbA1c was 7.80 ± 1.36% as compared with 7.62 ± 1.30% in patients with newly diagnosed DM (p < 0.001). These patients were younger (62.0 ± 11.3 years) compared to DM (67.9 ± 10.4 years) and non-DM (63.7 ± 13.0) patients, p < 0.001. 1-year MACCE rates were 14.8%, 19.5% and 27.96% in the non-DM, newly diagnosed DM and DM groups, respectively (p < 0.001). Multivariate analysis showed that compared to non-DM, the adjusted one-year hazard ratios for MACCE were 1.75 and 1.40 in patients with known DM and newly diagnosed DM, respectively (p < 0.05 for both). Conclusion Newly diagnosed DM based on peri-procedural HbA1c is common and associated with increased short and long term risk for adverse outcomes. Our results may warrant routine screening for DM in all patients undergoing PCI.