Prostaglandin (PG)E2 is a major cyclooxygenase (COX) product that is important in human physiology and pathophysiology. Quantification of systemic PG production in humans is best assessed by measuring excreted urinary metabolites. Accurate and easy-to-perform assays to quantify the major urinary metabolite of PGE2, 11alpha-hydroxy-9,15-dioxo-2,3,4,5-tetranor-prostane-1,20-dioic acid (PGE-M), do not exist. We now report the development of a robust and facile method to measure urinary PGE-M excretion in humans using stable isotope dilution techniques employing liquid chromatography/tandem mass spectrometry (LC/MS/MS). Concentrations of the metabolite in urine from healthy humans are nearly twofold greater in men than in women (10.4+/-1.5 vs. 6.0+/-0.7 ng/mg creatinine). Levels of PGE-M in healthy humans are suppressed significantly not only by the nonselective COX inhibitor ibuprofen but also by the COX-2 selective inhibitor rofecoxib, suggesting that the majority of PGE2 formed in vivo is derived from COX-2. Increased COX-2 expression and increased PGE2 production are associated with malignancy. Levels of PGE-M were found to be greatly increased in humans with unresectable non-small cell cancer of the lung, and this increase is dramatically reduced by administration of the COX-2 inhibitor celecoxib, implying that COX-2 contributes significantly to the overproduction of PGE2. In summary, quantification of PGE-M using LC/MS/MS provides a facile and accurate method to assess PGE2 formation in human physiological and pathophysiological processes.