BACKGROUND The relationships between culprit coronary plaque characteristics and hyperhomocysteinemia (HHcy) are not fully understood in young patients. In this study we investigated the relationship between culprit atherosclerotic plaque phenotype assessed by optical coherence tomography (OCT) and hyperhomocysteinemia (HHcy) in young patients. MATERIAL AND METHODS We investigated the OCT imaging and HHcy of 123 lesions in 123 young patients (≤45 years of age). According to OCT images, culprit lesions were classified as thin-cap fiber atheroma (TCFA), thrombus, and other. The 123 patients were grouped as: HHcy group (53 cases, HHcy ≥15.5 µmol/l) and control group (70 cases, HHcy <15.5 µmol/l). RESULTS Compared with the control group, the HHcy group had a higher proportion of OCT-TCFA (p=0.03), OCT-vasa vasorum (p=0.013), and OCT-thrombus (p=0.012), and a larger lipid arc (p=0.002). HHcy (P=0.037) and metabolic syndrome (MetS) (P=0.016) remained independent predictors of TCFAs. HHcy (P=0.026) and smoking (P=0.005) remained independent determinants of thrombus. CONCLUSIONS HHcy and MetS are associated with TCFAs, and HHcy and smoking are associated with thrombus in young patients with coronary artery disease.