Introduction: International Classification of Diseases (ICD)-10 codes are routinely used to classify patients with heart failure (HF). Although these codes play an important role with case identification and reimbursement, they are perceived to lack clinical specificity.Goals/Aims: We sought to describe the relationship between HF type (based on left ventricular ejection fraction [LVEF]) and assigned HF ICD-10 code in a large real-world dataset.Methods: We performed a retrospective cross-sectional analysis of patients admitted to a large health system within the western US between 1/1/18 and 10/1/2022 with a principal diagnosis of HF (ICD-10 codes: I50.2, systolic HF; I50.3, diastolic HF; I50.4, combined systolic and diastolic HF; I11.0, hypertensive heart disease with HF; and I13.0 and I13.2, hypertensive heart disease with HF and chronic kidney disease [CKD]). The analysis was further limited to patients with an LVEF by echocardiography in the 3 months preceding hospital discharge.Results: Over nearly 5 years, 61,238 HF hospitalizations occurred, of which 49,772 (81%) had an LVEF available. Most patients admitted with systolic HF (4.4% of the cohort) as well as systolic and diastolic HF (3.2% of the cohort) had an LVEF ≤40% (86.2% and 74.8%, respectively); similarly, most patients admitted with diastolic HF (3.1% of the cohort) had an LVEF ≥50% (94.0%) (Figure). Those admitted with hypertensive heart disease with HF (36.3% of the cohort) and hypertensive heart disease with HF and CKD (52.9% of the cohort) had a much wider LVEF distribution (Figure).Conclusions: Although relatively good concordance was noted between LVEF and ICD-10 codes for systolic HF, diastolic HF, and systolic and diastolic HF, this was not the case for most HF hospitalizations. This should be considered when performing analyses with HF ICD-10 codes.