Fiberoptic bronchoscopy with bronchoalveolar lavage (BAL) has become a crucial tool in the management of lung transplant recipients. Detection of pulmonary infectious pathogens by culture, cytology, and histology of BAL, protected brush specimens, and transbronchial biopsies (TBB) is highly effective. Morphologic and phenotypological analyses of BAL cells may be suggestive for certain complications after lung transplantation. For interpretation of BAL findings, the natural course of BAL cell morphology and phenotypology after lung transplantation must be considered. During the first 3 months after pulmonary transplantation, elevated total cell count in BAL and neutrophilic alveolitis are common, representing the cellular response to graft injury and interaction of immunocompetent cells of donor and recipient origin. With increasing time after transplantation the CD4/CD8 ratio decreases due to lowered percentages of CD4 cells in BAL. During bacterial pneumonias, the cellular profile of BAL is characterized by a marked granulocytic alveolitis. Lymphocytic alveolitis with a decreased CD4/CD8 ratio is suggestive of acute rejection, but is also found in viral pneumonias and obliterative bronchiolitis. In the case of a combined lymphocytosis and neutrophilia without any evidence of infection, obliterative bronchiolitis should be considered. Functional analyses of BAL cells can give additional information about the immunologic status of the graft, even before histologic changes become evident but have not been established in routine transplant monitoring. However, functional studies suggest an important role of activated, alloreactive and donor-specific T lymphocytes in the pathogenesis of acute and chronic lung rejection. Investigations of soluble components in BAL have given further insight into the immunologic processes after lung transplantation. In this overview, the characteristics of BAL after lung transplantation will be summarized, and its relevance for the detection of pulmonary complications will be discussed.