Background Cisplatin based chemotherapy is a well recognized risk factor for coagulation disordrs and thrombosis.The pathophysiological mechanisms by which cisplatin promote thrombosis are not well understood.Methods Red blood cells (RBCs) were separated from peripheral blood of patients with breast cancer (n=10) and healthy adults (n=6) and treated with cisplatin. Coagulation time of RBCs was assessed by one step recalcification time and the productions of thrombin, intrinsic and extrinsic factor Xa were measured in the presence or absence of various concentrations of lactadherin. Exposed phosphatidylserine was stained with lactadherin and observed by confocal microscopy and flow cytometry.Results Neither fresh RBCs nor RBCs treated without cisplatin had potent procoagulant activity. Cisplatin treatment increased procoagulant activity of RBCs in a cell number- and concentration-dependent manner. Exposed phosphatidylserine was stained with lactadherin and after cisplatin treatment, strong fluorescence was revealed by confocal microscopy. Lactadherin bound RBCs from patients with breast cancer increased from (1.9±0.5)% on control RBCs to (68.0±3.5)% on RBCs treated with 10 pmol/L cisplatin for 24 hours.Conclusions Cisplatin treatment increases procoagulant activity of RBCs, which have a strong association with exposure of phosphatidylserine. The increased procoagulant activity may contribute to the pathogenesis of thrombophilia during cisplatin based chemotherapy in breast cancer patients.