We hypothesize that prostacyclin (PGI 2 ) is an anti-atherosclerotic hormone and that atherosclerosis develops when endothelial PGI 2 synthetase is inhibited by lipid peroxides. Serum lipid peroxides occur in low-density lipoproteins (LDL). LDL lipid peroxides are elevated in common types of hyperlipoproteinaemias, PGI 2 generation is impaired in atherosclerosis, and infusion of synthetic PGI 2 into patients alleviates symptoms resulting from arteriosclerosis obliterans, central retinal vein occlusion or spontaneous angina.