Central venous plasma concentrations of 6-keto-prostaglandin F1 alpha (PGF1 alpha), the stable hydrolysis product of prostacyclin (PGI2, a vasodilator and antiaggregatory metabolite of arachidonic acid), were determined in patients with septic shock. In eight nonsurvivors, the median plasma 6-keto-PGF1 alpha level was 229 pg/ml (range 31 to 21,998), compared to 30 pg/ml (range 22 to 194) in six survivors. In three control patients who were not septic or in shock, the levels were less than 4 pg/ml. This study demonstrates that human septic shock is associated with elevated plasma levels of 6-keto-PGF1 alpha, and raises the possibility that increased PGI2 formation may play a role in human septic shock.