Background: Materials extracted from atherosclerotic arteries can disclose data about the molecular pathology of cardiovascular disease, but obtaining such samples is complex and requires invasive surgery. To overcome this barrier, this study investigated whether angioplasty balloons inflated during standard percutaneous coronary interventions retain proteins from treated (dilated) atherosclerotic lesions and whether proteomic analysis of this material could provide data on lesion protein profiles and distinguish between patients with stable and unstable coronary artery disease. Methods: Patients with ST-segment–elevation myocardial infarction and stable angina pectoris were subjected to routine percutaneous coronary interventions. All angioplasty balloons inflated in a coronary artery were collected. Proteins retained on the balloons were extracted and analyzed using shotgun proteomic analysis. Results: Proteomics identified and quantified 1365 unique proteins captured on percutaneous coronary intervention balloons. Control balloons inflated in the ascending aorta showed minimal nonspecific protein binding, indicating specificity to the luminal region of atherosclerotic lesions of the diseased artery wall. Clustering and principal component analyses showed that ST-segment–elevation myocardial infarction and stable angina pectoris subjects could be separated by variations in protein content and abundance. We identified 206 proteins as differentially abundant between ST-segment–elevation myocardial infarction and stable angina pectoris subjects. Pathway analysis indicated several enriched processes in the ST-segment–elevation myocardial infarction group involved in neutrophil-mediated immunity and platelet activation. Conclusions: Disease-related proteins from coronary artery lesions adhere to angioplasty balloons and constitute a source of material for proteomic analysis. This approach can identify proteins and processes occurring in unstable coronary atherosclerotic lesions and suggest novel therapeutic approaches.