This article has outlined the features of the major types of infections encountered in pulmonary allograft recipients. Virtually any pathogen can cause infection in these immunocompromised subjects, and there is a distinct propensity for these organisms to invade the transplanted lung. As is the case with other major organ recipients, there is a temporal sequence in the types of infection lung allograft recipients contract. Bacteria are the most frequent cause of pneumonia in the first postoperative month. After this period, infection with CMV, particularly CMV pneumonitis, becomes most common. Following the "window" for CMV infection, the risk for infestation with P. carinii becomes the primary concern. The latter two types of infection pose a double risk for the recipient: (1) morbidity and mortality from the infection itself and (2) chronic rejection following on the heels of these infections and producing morbidity and mortality on its own. Pulmonary allograft recipients are also susceptible to fungi, particularly C. albicans. Although these infections rarely produce an overwhelming pneumonia, they nonetheless carry a grave prognosis because they usually become widely disseminated. Better selection of donor lungs and prophylactic measures such as the use of broad-spectrum antibiotics and amphotericin B in the early postoperative period, the use of CMV-negative blood products in seronegative recipients, and the chronic administration of trimethoprim-sulfamethoxazole have reduced the rate of infection with bacteria, fungi, CMV (primary infections only), and P. carinii, respectively. Despite these relative successes, however, the risk for infection of the allograft remains high because the defense mechanisms in the lung allograft are breached by the effects of surgery, the "allogeneic environment" in the allograft and systemic immunosuppression, and the fact that chronic rejection causes structural changes that predispose to bacterial colonization of the airways and the need for increased levels of immunosuppression. Despite the formidable barrier that infection of the lung allograft poses, the procedure of pulmonary transplantation clearly holds sufficient promise that all efforts possible should be made to hurdle this barrier. Achieving such a goal would ensure a place for pulmonary transplantation in the armamentarium of treatment for irreversible pulmonary disease.