Background and Aims. Lowering the blood concentration of low-density lipoprotein cholesterol (LDL-C), is a cornerstone in preventing atherosclerotic cardiovascular disease (ASCVD). Current European guidelines recommends LDL-C < 1.4 mmol/L for secondary prevention in high-risk patients. The aim of this study is to investigate monitoring and treatment of hypercholesterolemia one year after a ASCVD event. Methods: Danish patients with hypercholesterolemia and an incident ASCVD event from 2015 to 2020 were included in this nationwide cohort study. Patients' LDL-C measurements and lipid-lowering treatment were followed for one year after ASCVD event, or until death or migration. Imputation was used to estimate absolute LDL-values when patients were unmeasured. Results: A total of 139,043 patients were included in the study with a mean follow-up time of 10.4 months. During the one-year period, 120,020 (86%) patients had their LDL-C measured at least once, 83,723 (60%) patients were measured at least twice. During the period one to six months after ASCVD event 25,999 (19%) achieved an LDL-C < 1.4 mmol/L, 93,349 (67%) failed to achieve an LDL-C < 1.4 mmol/L, and 196,950 (14%) had died or migrated. Missing LDL-C values were estimated via imputation. At the end of month twelve, 60,583 (44%) patients were in statin monotherapy, 2926 (2%) were treated with other lipid-lowering treatment, 42,869 (31%) were in no treatment, and 32,665 (23%) had died or migrated. Conclusions: Many Danish patients are not appropriately followed-up with LDL-C measurements, and a substantial number of patients are not in lipid-lowering treatment one year after an ASCVD event.