Abstract. Iron salts are used in the treatment of iron deficiency anemia. Diabetic patients are frequently anemic and treatment includes administration of iron. Anemic patients on hemodialysis are at an increased risk of thromboembolic coronary events associated with the formation of dense fibrin clots resistant to fibrinolysis. Moreover, in chronic kidney disease patients, high labile plasma iron levels associated with iron supplementation are involved in complications found in dialyzed patients such as myocardial infarction. The aim of the present study was to investigate whether iron treatment is involved in the formation of the fibrin clots. Clotting of citrated plasma supplemented with Fe 3+ was investigated by thromboelastometry and electron microscopy. The results revealed that iron modifies coagulation in a complex manner. FeCl 3 stock solution underwent gradual chemical modification during storage and altered the coagulation profile over 29 days, suggesting that Fe 3+ interacts with both proteins of the coagulation cascade as well as the hydrolytic Fe 3+ species. Iron extends clotting of plasma by interacting with proteins of the coagulation cascade. Fe 3+ and/or its hydrolytic species interact with fibrinogen and/or fibrin changing their morphology and properties. In general FeCl 3 weakens the fibrin clot while at the same time precipitating plasma proteins immediately after application. Fe 3+ or its derivatives induced the formation of insoluble coagulums in non-enzymatic reactions including albumin and transferrin. Iron plays a role in coagulation and can precipitate plasma proteins. The formation of coagulums resistant to lysis in non-enzymatic reactions can increase the risk of thrombosis, and extending clotting of plasma can prolong bleeding.