We report a case of severe respiratory failure due to cytomegalovirus pneumonitis in a patient who underwent an allogeneic bone marrow transplant, who was successfully treated with the combination of ganciclovir and high-dose intravenous immune globulin. We also reviewed the rationale for the use of combination therapy with an antiviral agent and immunotherapy. Because of the bone marrow toxicity of ganciclovir, an aggressive diagnostic approach, including bronchoalveolar lavage and open lung biopsy, may be necessary to establish a definitive diagnosis prior to institution of therapy.