Introduction YKL‑40 is a protein released locally by inflammatory cells. Thus, it may constitute a biomarker of inflammatory conditions, such as atherosclerosis. Objectives The aim of the study was to determine YKL‑40 levels in patients with ischemic heart disease and to analyze the correlation of this biomarker with the severity of coronary atherosclerosis. Patients and methods The study included 158 patients: 52 with stable ischemic heart disease and 67 with acute coronary syndrome: ST‑segment elevation myocardial infarction (STEMI; n = 47) or non-ST‑segment elevation myocardial infarction (NSTEMI; n = 20). The control group included 39 individuals without abnormalities in coronary vessels. We evaluated plasma YKL‑40 levels and their correlation with the severity of coronary atherosclerosis assessed with the SYNTAX score. Results Patients with myocardial infarction had higher plasma YKL‑40 levels than those with stable ischemic disease (median [range], 235.3 [161.6-366.1] ng/ml vs 61.2 [53.1-83.1] ng/ml; P <0.001) or controls (median [range], 235.3 [161.6-366.1] ng/ml vs 55.7 [51.2-75.2] ng/ml; P <0.001). No differences were found in YKL‑40 concentrations between STEMI and NSTEMI patients (median [range], 263 [150.3-363.7] ng/ml and 214.9 [163.4-367.6] ng/ml, respectively; P = 0.7). The SYNTAX score in patients with ischemic heart disease correlated positively with YKL‑40 concentrations (R = 0.34; P <0.001). Conclusions YKL‑40 can be considered a potential biomarker of coronary atherosclerosis severity.