Abstract Hematologic dysfunction occurs commonly in patients with malignancy. Over half are anemic, often because of acute or chronic blood loss, marrow involvement by the malignancy, marrow suppressive effects of chemotherapy or radiation therapy, or because of the anemia of chronic disease. Less frequently, anemia may result from red cell aplasia, folate or B12 deficiency, hemolytic processes, or hypersplenism. Occasional patients may become polycythemic because of erythropoietin-producing tumors such as renal adenocarcinomas or cerebellar hemangiomas. Elevation of the white cell count is commonly seen, especially in patients with lung cancer. Monocytosis and thrombocytosis, which may be early signs of an underlying malignancy, are also very common and occur in up to half of patients. Thrombocytopenia is commonly a result of therapy or marrow replacement; a few patients may have a syndrome resembling immune thrombocytopenic purpura. Abnormalities of coagulation are present in many patients, and may lead to superficial or deep venous thromboses, pulmonary emboli, nonbacterial thrombotic endocarditis with arterial emboli, bleeding, or acute disseminated intravascular coagulation. A sound understanding of the potential hematologic complications that can result from the malignant process is essential to the clinician caring for cancer patients.