Patients with hematologic malignancies have increased risks of thromboembolism or bleeding. The commonest thrombotic complication is venous thromboembolism (VTE). Other thrombotic conditions occur in association with specific disorders, such as thrombotic thrombocytopenic purpura in hematopoietic stem cell transplantation and disseminated intravascular coagulation in acute promyelocytic leukemia. Clinical trials show that unfractionated heparin (UFH) and low molecular weight heparin (LMWH) are efficacious for VTE prophylaxis in cancer patients after major surgery or when hospitalized for acute medical illnesses. These findings in cancer patients are probably applicable to patients with hematologic malignancies, in whom there are very few studies. However, the effectiveness of anticoagulant VTE prophylaxis is not established in ambulatory patients with cancer except for multiple myeloma patients treated with thalidomide and chemotherapy. LMWH is widely used as initial treatment for VTE because it enables home therapy without laboratory monitoring, thereby improving the patient's quality of life. UFH is preferred in patients with high bleeding risks and renal impairment. In cancer patients, vitamin K antagonists for the long-term treatment of VTE are increasingly replaced by LMWH, which show superior efficacy. When prescribing anticoagulant prophylaxis or treatment to patients with hematologic malignancies, clinical benefits must be weighed carefully against the risks of bleeding.