Platelets are usually considered as initiators of thrombosis and hemostasis in damaged endothelium and disintegrated blood vessels. However certain researches of the last ten years have shown that platelets take active part in the pathogenesis of tumor diseases contributing to the development of metastases and neoangiogenesis as well as providing tumor cells with various growth factors. The present review deals with the link between the changes in peripheral blood (number of thrombocytes, mean thrombocyte volume) and the course of tumor disease. We have analyzed the interaction between the platelets activated by tumor cells and the immune cells that let the tumor escape the immune surveillance. Here, data are presented on the pharmacological agents capable of inhibiting the activation of platelets and thus preventing the metastatic progression of tumor disease.