Marrow transplantation is now an accepted form of therapy for patients with aplastic anemia, genetic diseases, and a variety of malignant diseases. Intensive chemotherapy and radiotherapy regimens are necessary to eradicate cancer cells and allow engraftment of the transplanted marrow. Patients undergoing such treatment suffer prolonged marrow aplasia and immunosuppression and experience significant nonmarrow toxicities. This requires maximum supportive care including hyperalimentation, fluid and blood transfusions, antibacterial and antiviral prophylaxis, and measures to reduce organ toxicities and accelerate engraftment. The use of drugs to suppress the production of antitumor necrosis factor-alpha has shown promise in reducing the nonmarrow toxicities of the conditioning regimen. Prophylactic antibiotics that reduce gastrointestinal colonization with aerobic bacteria yet preserve anaerobic flora may more effectively reduce not only systemic bacteria but fungal infections as well. The impact of infections due to cytomegalovirus has been reduced by more effective prevention in patients who are cytomegalovirus negative and reactivation in patients who are cytomegalovirus positive. The use of recombinant growth factors will significantly reduce morbidity after transplantation by reducing the period of marrow aplasia.