Tens of thousands of patients undergo hematopoietic stem cell transplantation (HSCT) annually, 15 to 40% of whom are admitted to the intensive care unit. Pulmonary complications are the most life threatening conditions that develop in HSCT recipients. Both infectious and noninfectious complications occur more frequently in allogeneic HSCT. The management of HSCT recipients requires knowledge of their immune status, appropriate diagnostic evaluation, and early treatment. During the preengraftment phase (0 to 30 days after transplant), the most prevalent pathogens causing infection are bacteria and Candida species and, if the neutropenia persists, Aspergillus species. The early postengraftment phase (30 to 100 days) is characterized by cytomegalovirus (CMV), Pneumocystis jiroveci, and Aspergillus infections. During the late posttransplant phase (> 100 days), allogeneic HSCT recipients are at risk for CMV, community-acquired respiratory virus, and encapsulated bacterial infections. Antigen and polymerase chain reaction assays are important for the diagnosis of CMV and Aspergillus infections. Diffuse alveolar hemorrhage (DAH) and periengraftment respiratory distress syndrome occur in both allogeneic and autologous HSCT recipients, usually during the first 30 days. Bronchiolitis obliterans occurs exclusively in allogeneic HSCT recipients with graft versus host disease. Idiopathic pneumonia syndrome occurs at any time following transplant. Bronchoscopy is usually helpful for the diagnosis of the infectious pulmonary complications and DAH.