Approximately 4% of recipients of solid organ transplants in the United States develop bacterial pneumonia in the posttransplant period, often in the first 3 months following transplantation. The incidence of bacterial pneumonia is highest in recipients of heartlung (22%) and liver transplants (17%), intermediate in recipients of heart transplants (5%), and lowest in renal transplant patients (1 to 2%). The crude mortality of bacterial pneumonia in solid organ transplantation has exceeded 40% in most series. Beyond those risk factors identified for nosocomial pneumonia, the occurrence of primary cytomegalovirus (CMV) infection, graft rejection, maintenance antirejection therapy with prednisone, azathioprine, and antilymphocyte globulin, antirejection therapy with high-dose corticosteroids or OKT3 and splenectomy have been associated with a significantly increased risk of bacterial pneumonia in these patients. In the first 3 months posttransplant, gram-negative bacilli, Staphylococcus aureus and Legionella predominate and mortality is very high, in excess of 60%. Thereafter, bacterial pneumonias are caused primarily by Streptococcus pneumoniae and Hemophilus influenzae, with considerably lower mortality. Bacterial pneumonia must be suspected in any transplant patient presenting with fever and cough, especially associated with dyspnea or infiltrates on chest radiograph. If large numbers of bacteria and polymorphonuclear leukocytes are not visualized in respiratory secretions the work-up should proceed directly to fiberoptic bronchoscopy with bronchoalveolar lavage and/or protected brush specimen to establish the microbiologic diagnosis as accurately as possible. For presumptive gram-negative bacillary pneumonia, the initial regimen must be effective against Pseudomonas aeruginosa. Prevention of bacterial pneumonia in transplant patients must begin with immunization against S pneumoniae and Influenza A, and include precautions taken to prevent nosocomial pneumonia. It further may include measures to prevent CMV infection and the use of trimethoprim/sulfamethoxazole prophylaxis during the first year posttransplantation. Ultimately, novel technologies such as selective antimicrobial decontamination and/or protective isolation during the early postoperative period may prove effective.