Twenty-six cases of lymphocytic bronchitis/bronchiolitis (LBB) identified by transbronchial biopsy in 25 lung allograft recipients were studied to determine its relationships to acute rejection and bronchiolitis obliterans (OB). LBB occurred 355 days after transplantation on average (range, 15-2,118 days) and was manifested by a patchy or diffuse submucosal infiltrate of lymphocytes and plasma cells, which percolated deep to the smooth muscle layer of the bronchi in 15 cases. Submucosal granulation tissue and bronchiolitis were observed more frequently in patients who developed OB than in those who did not (44% and 88% vs 23% and 41%). Although 39% of patients progressed to OB overall, the majority of patients with LBB who received augmented immunosuppressive therapy (steroids, antithymocyte globulin, or both) improved or stabilized their pulmonary function abnormalities. Interestingly, LBB was preceded by acute rejection in 20 of 26 instances, and LBB frequently persisted as a histologic finding after the initial diagnostic transbronchial biopsy. LBB appears to be related to previous acute rejection episodes and responds to augmented immunosuppressive therapy.