Background: L-carnitine can be metabolized to trimethylamine N-oxide (TMAO), a molecule that promotes atherogenesis through its interaction with macrophages and lipid metabolism. Objective: The aim of the present study was to assess whether L-carnitine supplementation may promote changes in selected serum biomarkers of atherosclerosis. Methods: Before the start, in the mid-point and after completing the 24-weeks supplementation protocol, fasting blood samples were taken from the antecubital vein. Plasma free L-carnitine and TMAO were determined by the UPLC/MS/MS method. Serum proteins were determined by the enzyme immunoassay method using commercially available kits. Total cholesterol, high-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol, and triglycerides have been determined using standard automatic analyzer. Results: L-carnitine supplementation elevated fasting plasma carnitine in the mid-point of our study and it remained increased until the end of supplementation period. Moreover, it induced tenfold increase in plasma TMAO concentration but did not affect serum C-reactive protein, interleukin-6, tumour necrosis factor-α, L-selectin, P-selectin, vascular cell adhesion molecule-1, intercellular adhesion molecule-1 or lipid profile markers. Conclusion: We demonstrated that -although oral L-carnitine supplementation significantly -increased plasma TMAO concentration, no lipid profile changes or other markers of adverse cardiovascular events were detected in healthy aged women over the period of 24 weeks.