2-Chlorohexadecanal (2-ClHDA), a 16-carbon chain chlorinated fatty aldehyde that is produced by reactive chlorinating species attack of plasmalogens, is elevated in atherosclerotic plaques, infarcted myocardium, and activated leukocytes. We tested the hypothesis that 2-ClHDA and its metabolites, 2-chlorohexadecanoic acid (2-ClHA) and 2-chlorohexadecanol (2-ClHOH), induce COX-2 expression in human coronary artery endothelial cells (HCAEC). COX-2 protein expression increased in response to 2-ClHDA treatments at 8 and 20 h. 2-ClHA also increased COX-2 expression following an 8 h treatment. Quantitative PCR showed that 2-ClHDA treatment increased COX-2 mRNA over 8 h, while 2-ClHA treatment led to a modest increase by 1 h and those levels remained constant over 8 h. 2-ClHDA led to a significant increase in 6-keto-PGF_1α release (a measure of PGI₂ release) by HCAEC. These data suggest that 2-ClHDA and its metabolite 2-ClHA, which are produced during leukocyte activation, may alter vascular endothelial cell function by upregulation of COX-2 expression.