Prostacyclin is a potent vasodilator and platelet inhibitor produced by vascular endothelium. Endogenous production of prostacyclin under physiologic conditions is extremely low, far below the capacity of vascular tissue to generate this substance in response to stimulation in vitro. This may reflect a low frequency or intensity of stimulation of prostacyclin production. We postulated that if prostacyclin does act as an endogenous platelet-inhibitory agent, it should be produced in greater amounts in a clinical setting in which platelet-vascular interactions are likely to be increased. To test this hypothesis, we examined prostacyclin biosynthesis in patients with severe atherosclerosis and evidence of platelet activation in vivo. Excretion of 2,3-dinor-6-keto-prostaglandin F1 alpha, a major urinary prostacyclin metabolite, was significantly higher in 9 patients with severe atherosclerosis and evidence of platelet activation (251 to 1859 pg per milligram of creatinine) than in 54 healthy volunteers (45 to 219 pg per milligram of creatinine; P less than 0.001). This difference represented an alteration in biosynthesis rather than in metabolism, since the fractional conversion of infused prostacyclin to the dinor metabolite was identical in both groups. Prostacyclin production may be low in healthy persons because there is almost no stimulus for its production but enhanced in patients with severe atherosclerosis as a consequence of platelet interactions with endothelium or other vascular insults. These observations are compatible with a role for prostacyclin as a local regulator of platelet-vascular interactions.