Most oncologic emergencies can be classified as metabolic, hematologic, structural, or side effects from chemotherapy agents. Tumor lysis syndrome is a metabolic emergency that presents as severe electrolyte abnormalities. The condition is treated with allopurinol or urate oxidase to lower uric acid levels. Hypercalcemia of malignancy is treated with aggressive rehydration, furosemide, and intravenous bisphosphonates. Syndrome of inappropriate antidiuretic hormone should be suspected if a patient with cancer presents with normovolemic hyponatremia. This metabolic condition usually is treated with fluid restriction and furosemide. Febrile neutropenia is a hematologic emergency that usually requires inpatient therapy with broad-spectrum antibiotics, although outpatient therapy may be appropriate for low-risk patients. Hyperviscosity syndrome usually is associated with Waldenström's macroglobulinemia, which is treated with plasmapheresis and chemotherapy. Structural oncologic emergencies are caused by direct compression of nontumor structures or by metastatic disease. Superior vena cava syndrome presents as neck or facial swelling and development of collateral venous circulation. Treatment options include chemotherapy, radiation, and intravenous stenting. Epidural spinal cord compression can be treated with dexamethasone, radiation, or surgery. Malignant pericardial effusion, which often is undiagnosed in cancer patients, can be treated with pericardiocentesis or a pericardial window procedure.