Parenteral nutrition has a central role in the supportive therapy of patients submitted to a BMT. A central catheter is mandatory for transfusions, antibiotic therapy and a proper nutrition. A good nutritional support contributes to maintain hydration, reduce lean body mass loss, increase patient comfort and improve survival in patients who can not eat or absorb for a prolonged period of time. After a BMT metabolic complications are frequent and require careful monitoring; in critical care patients, the major risks are electrolyte and glucose disturbances. Liver disease is a main metabolic complication of PN, but it can occur in any cancer patient due to therapy or to graft-versus-host disease. Its best prevention requires the avoidance of prolonged enteral fasting.