Prostaglandin D(2) (PGD(2)) is a cyclooxygenase (COX) product of arachidonic acid that activates D prostanoid receptors to modulate vascular, platelet, and leukocyte function in vitro. However, little is known about its enzymatic origin or its formation in vivo in cardiovascular or inflammatory disease. 11,15-dioxo-9alpha-hydroxy-2,3,4,5-tetranorprostan-1,20-dioic acid (tetranor PGDM) was identified by mass spectrometry as a metabolite of infused PGD(2) that is detectable in mouse and human urine. Using liquid chromatography-tandem mass spectrometry, tetranor PGDM was much more abundant than the PGD(2) metabolites, 11beta-PGF(2alpha) and 2,3-dinor-11beta-PGF(2alpha), in human urine and was the only endogenous metabolite detectable in mouse urine. Infusion of PGD(2) dose dependently increased urinary tetranor PGDM > 2,3-dinor-11beta-PGF(2alpha) > 11beta-PGF(2alpha) in mice. Deletion of either lipocalin-type or hemopoietic PGD synthase enzymes decreased urinary tetranor PGDM. Deletion or knockdown of COX-1, but not deletion of COX-2, decreased urinary tetranor PGDM in mice. Correspondingly, both PGDM and 2,3-dinor-11beta-PGF(2alpha) were suppressed by inhibition of COX-1 and COX-2, but not by selective inhibition of COX-2 in humans. PGD(2) has been implicated in both the development and resolution of inflammation. Administration of bacterial lipopolysaccharide coordinately elevated tetranor PGDM and 2,3-dinor-11beta-PGF(2alpha) in volunteers, coincident with a pyrexial and systemic inflammatory response, but both metabolites fell during the resolution phase. Niacin increased tetranor PGDM and 2,3-dinor-11beta-PGF(2alpha) in humans coincident with facial flushing. Tetranor PGDM is an abundant metabolite in urine that reflects modulated biosynthesis of PGD(2) in humans and mice.