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Background: Low levels of the endogenous amino acid L-homoarginine are a risk factor for cardiovascular morbidity and mortality. For individual risk prediction, commercially available test systems are mandatory. This study aims at formulating sex- and age-specific reference intervals of serum L-homoarginine determined with an ELISA., Methods: We determined reference intervals for serum L-homoarginine stratified by age and sex in a sample of 1285 healthy participants of the Study of Health in Pomerania (SHIP)-TREND cohort after exclusion of participants with cardiovascular diseases, diabetes mellitus, hypertension, metabolic syndrome, elevated liver enzymes, chronic kidney disease stages III or IV, or body mass index >25 kg/m2. Serum L-homoarginine was determined applying a commercially available ELISA., Results: The reference cohort included 836 women (median age 41, 25th and 75th percentiles are 32 and 50 years) and 449 men (median age 38, 25th, and 75th percentiles are 30 and 49 years). The median serum concentration of L-homoarginine was 1.93 (25th 1.49; 75th 2.60) µmol/L in women and 2.02 (25th 1.63; 75th 2.61) µmol/L in men (P = 0.04). The reference intervals (2.5th to 97.5th percentile) were 0.89-5.29 µmol/L for women and 1.09-3.76 µmol/L for men. The L-homoarginine serum concentration declined over age decades in both sexes and a notable interaction with sex hormone intake in women was observed., Conclusions: The novelty of our study is that we determined reference intervals specific for the L-isomer being lower than those previously reported for homoarginine in SHIP and thus might be helpful in identifying individuals suitable for oral L-homoarginine supplementation., Competing Interests: Authors’ Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Disclosures and/or potential conflicts of interest: Employment or Leadership: R. Wesemann is managing director of WESAMIN GmbH & Co. KG, Winsen, Germany, responsible for the development and production of the DLD Diagnostika ELISA kit for L-homoarginine. Consultant or Advisory Role: None declared. Stock Ownership: None declared. Honoraria: C.-u.Choe received honoraria from Zambon GmbH. Research Funding: This work was funded in part by a grant from the DZHK (Deutsches Zentrum für Herz-Kreislauf-Forschung e.V., E. Schwedhelm, T. Ittermann, N. Friedrich). The SHIP-TREND study is part of the Community Medicine Research net of the University of Greifswald, Germany, which is funded by the Federal Ministry of Education and Research (Grants 01ZZ9603, 01ZZ0103, and 01ZZ0403), the Ministry of Cultural Affairs as well as the Social Ministry of the Federal State of Mecklenburg-West Pomerania (T. Ittermann, N. Friedrich). C.-u.Choe was supported by an Else Kröner Exzellenzstipendium from the Else Kröner-Fresenius Stiftung. Expert Testimony: None declared. Patents: None declared., (© American Association for Clinical Chemistry 2022. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)