Background: Inflammatory processes contribute to the development of atherosclerosis. However, it remains unclear whether immunoglobulin G4 (IgG4)-related immuno-inflammation may affect the progression of cardiovascular disease. We therefore analyzed the relationship between serum IgG4 level and future major adverse cardiovascular events (MACE) after percutaneous coronary intervention (PCI). Methods: We measured serum levels of IgG4 in 562 patients (mean age, 66.9 ± 10.3 years) who underwent coronary angiography from October 2005 to September 2008. Furthermore, a total of 108 patients who subsequently received first PCI were followed-up for a median period of 2721 days (interquartile range [IR] 1591-3183). MACE was defined as cardiac death, acute coronary syndrome, cerebral stroke, and target vessel revascularization. Results: Serum levels of IgG4 were significantly higher in patients with coronary artery disease (34.1 mg/dL, IR 18.7-58.9) than in those without (30.5 mg/dL, IR 15.8-50.3, P=0.045). Among patients who received PCI, 40 (37.0%) patients developed MACE during the follow-up period. Patients with serum IgG4 levels of median or greater (≥37.2 mg/dL) had a significantly higher risk of MACE than those with serum IgG4 levels below median (log-rank test, P=0.017). In Cox regression analysis using age, gender, estimated glomerular filtration rate (eGFR), hypertension, dyslipidemia, diabetes, smoking, and serum levels of high-sensitivity C-reactive protein (hsCRP) as covariates, serum IgG4 level was an independent predictor of MACE (hazard ratio 2.39 per 100 mg/dL increase, 95% confidence interval 1.41-4.07, P=0.001). Conclusions: Elevated serum IgG4 level at baseline significantly predicted MACE after PCI, as well as present coronary stenosis, over long-term follow-up even among patients without apparent IgG4-related disease. IgG4-related immuno-inflammation may play a certain role in the progression of coronary atherosclerosis.