Using the recent standardization of the pathologic definitions for acute lung rejection, we prospectively evaluated 66 consecutive bronchoalveolar lavage (BAL) and transbronchial biopsy (TBB) specimens in 32 patients after lung transplantation. Clinical indications for bronchoscopies were surveillance (n = 44), rejection (n = 18), and infection (n = 4). Bronchoalveolar lavages were obtained from the right middle lobe or lingula in single lung transplant and from both sites in double lung transplant recipients. Cytosmears for differential cell counts were performed and 400 to 500 cells were counted. Five to eight TBB specimens were taken from two different lobes and stained with hematoxylin-eosin, elastic trichrome, and silver methenamine. Sixty-four of 66 sets of biopsy specimens were satisfactory, but 3 were eliminated because of presence of cytomegalovirus cytopathic changes. Of the remaining 61, rejection was presented in 45 (74 percent): grade 1 in 23 (38 percent), grade 2 in 19 (31 percent), and grade 3 in 3 (5 percent). In 30 of 42 (71 percent) surveillance biopsy specimens, rejection was present, grade 1 in 18 (43 percent) and grade 2 or 3 in 12 (28 percent). In TBBs performed for clinical suspicion of rejection, 15 of 18 TBB specimens (83 percent) showed rejection, grade 1 in 5 (28 percent) and grade 2 or 3 in 10 (55 percent). Of four biopsies performed for suspicion of infection, one was normal and three showed rejection in addition to infection. These three were eliminated from further analysis due to the limitation of the Lung Rejection Study Group criteria in distinguishing rejection from infection. Of the 45 episodes of rejection, 24 (53 percent) occurred during the first 3 months posttransplantation, 8 (18 percent) between 3 and 6 months and 13 (29 percent) after 6 months. Percentage of BAL lymphocytosis was significantly elevated in grade 2 or 3 rejection (28 +/- 4) when compared with grade 1 (15 +/- 3) or grade 0 (10 +/- 3) (p < 0.001). Bronchoalveolar lavage lymphocytosis also correlated with severity of rejection (r = 0.6). We conclude that according to the standardized criteria of the Lung Rejection Study Group, acute lung rejection occurs more frequently than clinically suspected early and late after transplantation and that BAL lymphocytosis correlates with the presence and severity of histologically proven rejection.