Prostaglandins (PGs) are arachidonic acid (AA) derivatives via the PG endoperoxyde H synthase (PGHS) complex. Two PGHS isoforms have been recognized, constitutive (PGHS-1) and inducible (PGHS-2), respectively. Within the kidney, vascular endothelium mainly produces PGI2; the whole glomerulus synthesizes several prostanoids, the predominant AA metabolite in humans being PGI2; tubules and medullary interstitial cells produce mainly PGE2. Renal PGs modulates the action of other hormones and autacoids involved in the regulation of renal hemodynamics, glomerular filtration and the renal handling of sodium and water. Renal PGs are, at least in part, excreted into urine. Measurement of urinary PGs or their metabolites has been found to provide a reliable estimation of basal as well as stimulated PG synthesis. Patients with cirrhosis of the liver show an increased renal synthesis of vasodilating PGs, as indicated by the high urinary excretion of PGs and/or their metabolites. Administration of nonsteroidal anti-inflammatory drugs (NSAIDs) to these patients causes a profound reduction in renal blood flow and glomerular filtration rate, a reduction in sodium excretion, and an impairment of free water clearance. These data clearly indicate that the increased renal synthesis of vasodilating PGs has a relevant role in maintaining renal hemodynamics, sodium and water excretion in a clinical setting characterized by a reduction of effective plasma volume and a striking activation of the major vasoconstricting systems, namely the renin-angiotensin-aldosterone, the sympathetic nervous system, and vasopressin. Patients with hepato-renal syndrome have a reduced renal synthesis of vasodilating PGE2 in the setting of a striking activation of endogenous vasoconstrictors and a maintained or increased renal production of thromboxane A2. Therefore, an imbalance between vasoconstricting systems and the renal vasodilator PGE2 was proposed to explain the renal failure observed in this condition. The urinary excretion of 2-3-dinor 6-keto-PGF1 alpha, an index of systemic PGI2 synthesis, is increased in patients with cirrhosis and hyperdynamic circulation, thus raising the possibility that systemic synthesis of PGI2 may contribute to the arterial vasodilatation of these patients. Finally, administration of exogenous prostanoids to patients with cirrhosis is not effective either in ameliorating renal function or in preventing the deleterious effect of NSAIDs.