: Atherosclerotic cardiovascular disease is one of the most common causes of death and disability around the world. Hypercholesterolemia is an established and widely prevalent risk factor; however, the prevalence of severe hypercholesterolemia (which is characteristic for familial hypercholesterolemia) has been studied far less. The aim of this study was to determine the prevalence of severe hypercholesterolemia among patients with acute coronary syndrome. : A retrospective study of patients hospitalised at Vilnius University Hospital Santaros Klinikos due to acute coronary syndrome was performed. Data were attained from an electronic medical history database. Data such as sex, age, cardiovascular risk factors (hypertension, diabetes) and low-density cholesterol results were collected. Severe hypercholesterolemia was defined as low-density lipoprotein cholesterol levels ≥ 4.9 mmol/L. : A total of 34,669 patients were included in this study (12,115 females (34.9%) and 22554 (65.1%) males, < 0.001). The median age of the entire study population was 67 years. A total of 3434 patients (9.9%) had severe hypercholesterolemia, 371 (1.1%) patients met the criteria for phenotypically probable familial hypercholesterolemia, and 36 (0.1%) patients presented with phenotypically definite familial hypercholesterolemia. The most common concomitant risk factor in this study was arterial hypertension, which was found in 48% of patients. : Based on the results of this study, severe hypercholesterolemia is prevalent among patients with acute coronary syndrome, with as many as 9.9% of patients presenting with severe hypercholesterolemia at the time of hospitalisation. The definite familial hypercholesterolemia phenotype is scarcer, with prevalence reaching 0.1% of patients with acute coronary syndrome.