Viral respiratory infections are uncommon causes of pulmonary infiltrates in immunocompetent patients who undergo cardiothoracic surgery. In winter months, however, influenza can be acquired in the community preoperatively or in the hospital setting. The recognition of influenza cases is essential to prevent nosocomial transmission. Respiratory syncytial virus is also an important pulmonary pathogen in pediatric patients who undergo cardiothoracic surgery and may produce serious disease in children with underlying pulmonary or congenital heart disease. Viral infections of the respiratory tract are important causes of morbidity and mortality in heart and lung transplant recipients, especially cytomegalovirus (CMV). Other herpes viruses such as varicella zoster virus and herpes simplex virus may also occasionally involve the lung. Epstein-Barr virus has been incriminated in the pathogenesis of post-transplant lymphoproliferative disease, an uncommon but severe complication of transplantation. Except for Epstein-Barr virus, effective therapy exists for CMV and the other herpes viruses. Prophylaxis with ganciclovir is effective in preventing serious CMV infections in seropositive heart transplant recipients. However, better strategies are needed to prevent primary CMV infection in these patients.