The rationale of the Czech Hematological Society guidelines for diagnosis and treatment of Philadelphia chromosome-negative myeloproliferative disorders with thrombocythemia (MPD-T) is reviewed. For diagnosis of MPD-T, the classification according to the World Health Organization or to the Rotterdam criteria is preferred because they distinguish true essential thrombocythemia from prefibrotic or early fibrotic idiopathic myelofibrosis and prepolycythemic polycythemia vera. The histopathology-based nosological distinction provided by these classifications yields valuable information on prognosis (including the risks of transition into secondary acute myeloid leukemia and myelofibrosis). Another serious complication in MPD-T is thrombosis (arterial or venous), the main risk factors of which are age, previous thrombosis, platelet counts 350 to 2,200 x 10 (9)/L (peak at approximately 900 x 10 (9)/L) and the presence of additional thrombophilic risk factors (hereditary thrombophilia, any hypercoagulable state, cardiovascular disease). The hemorrhagic risk starts increasing progressively at platelet counts > 1,000 x 10 (9)/L. Treatment should be stratified with respect to the thrombotic and hemorrhagic risks. In high-risk patients, thromboreductive therapy is warranted. All of the cytostatic drugs, including hydroxyurea, may be leukemogenic and should be given only to patients > 60 years old, whereas anagrelide or interferon alpha are preferred in younger individuals. In low-risk patients, antiaggregation therapy is sufficient, unless the platelet count exceeds 1,000 x 10 (9)/L, which is another indication for thromboreduction. Thrombopheresis is indicated in thrombocythemia > 2,000 x 10 (9)/L.