Over a four-month period (November 1991-February 1991), six patients underwent lung transplantation for end-stage pulmonary disease. Twenty-four transbronchial biopsies with concurrent bronchoalveolar lavage were performed at regular intervals to evaluate acute graft rejection and/or infection. A study was performed to evaluate the role of lavage in the cytodiagnosis of acute pulmonary graft rejection. Histologic features of acute rejection were present in 11/24 biopsies (46%), consisting of perivascular and/or peribronchial lymphoid aggregates. Concurrent lavage findings were hyperplastic alveolar lining cells (one specimen, 9%), atypical lymphocytes (one specimen, 9%), and acute and chronic inflammation (nine specimen, 82%). Among the 13 lavages without concurrent histologic evidence of acute graft rejection (54%), atypical lymphocytes were present in one specimen (8%). The majority of the specimens (54%) showed a relative lack of inflammation. Two of the 24 lavages (8%) contained either Candida or cytomegalovirus (CMV), which were not present on the concurrent biopsy, and one biopsy contained CMV, which was absent on the lavage smear. Although a higher percentage of lavages with histologic evidence of acute graft rejection contained acute and chronic inflammatory cells as compared to lavages without concurrent histologic evidence of rejection, the cytodiagnosis of acute graft rejection on bronchoalveolar lavage is indeterminate.