12-Hydroxy-12-octadecanoylcarnitine is an acylcarnitine. More specifically, it is an 12-hydroxyoctadecanoic acid ester of carnitine. Acylcarnitines were first discovered more than 70 year ago (PMID: 13825279). It is believed that there are more than 1000 types of acylcarnitines in the human body. The general role of acylcarnitines is to transport acyl-groups (organic acids and fatty acids) from the cytoplasm into the mitochondria so that they can be broken down to produce energy.  This process is known as beta-oxidation. According to a recent review [Dambrova et al. 2021, Physiological Reviews], acylcarnitines (ACs) can be classified into 9 different categories depending on the type and size of their acyl-group: 1) short-chain ACs; 2) medium-chain ACs; 3) long-chain ACs; 4) very long-chain ACs; 5) hydroxy ACs; 6) branched chain ACs; 7) unsaturated ACs; 8) dicarboxylic ACs and 9) miscellaneous ACs. Short-chain ACs have acyl-groups with two to five carbons (C2-C5), medium-chain ACs have acyl-groups with six to thirteen carbons (C6-C13), long-chain ACs have acyl-groups with fourteen to twenty once carbons (C14-C21) and very long-chain ACs have acyl groups with more than 22 carbons. 12-Hydroxy-12-octadecanoylcarnitine is therefore classified as a long chain AC. As a long-chain acylcarnitine 12-hydroxy-12-octadecanoylcarnitine is generally formed through esterification with long-chain fatty acids obtained from the diet. The main function of most long-chain acylcarnitines is to ensure long chain fatty acid transport into the mitochondria (PMID: 22804748). Altered levels of long-chain acylcarnitines can serve as useful markers for inherited disorders of long-chain fatty acid metabolism. In particular 12-hydroxy-12-octadecanoylcarnitine is elevated in the blood or plasma of individuals with coronary artery disease (PMID: 20173117). Carnitine palmitoyltransferase I (CPT I, EC:2.3.1.21) is involved in the synthesis of long-chain acylcarnitines (more than C12) on the mitochondrial outer membrane.  Elevated serum/plasma levels of long-chain acylcarnitines are not only markers for incomplete FA oxidation but also are indicators of altered carbohydrate and lipid metabolism. High serum concentrations of long-chain acylcarnitines in the postprandial or fed state are markers of insulin resistance and arise from insulin's inability to inhibit CPT-1-dependent fatty acid metabolism in muscles and the heart (PMID: 19073774). Increased intracellular content of long-chain acylcarnitines is thought to serve as a feedback inhibition mechanism of insulin action (PMID: 23258903). In healthy subjects, increased concentrations of insulin effectively inhibits long-chain acylcarnitine production. Several studies have also found increased levels of circulating long-chain acylcarnitines in chronic heart failure patients (PMID: 26796394). The study of acylcarnitines is an active area of research and it is likely that many novel acylcarnitines will be discovered in the coming years. It is also likely that many novel roles in health and disease will be uncovered. An excellent review of the current state of knowledge for acylcarnitines is available at [Dambrova et al. 2021, Physiological Reviews].