As antineoplastic therapy improves, patients with cancer will have the potential for prolonged survival. Tumor products may produce metabolic complications, such as hypercalcemia, hyponatremia, and hypoglycemia, and these conditions must be promptly recognized so that treatment can be initiated before severe or life-threatening symptoms occur. Tumor-lysis syndrome is a metabolic complication that occurs most frequently when effective treatment for rapidly proliferating tumors is initiated. It is a syndrome that can be prevented or ameliorated with appropriate anticipatory treatment. Recognition of metabolic emergencies common to cancer patients will improve quality of life and improve survival, allowing patients to benefit from definitive cancer treatment.