Chronic rejection in the form of obliterative bronchiolitis (OB) associated with chronic inflammation and fibrosis of bronchi (CIB) is a significant cause of morbidity and mortality in long-term heart-lung and single-lung transplant recipients. To determine the utility of transbronchial biopsy in diagnosing chronic rejection, we reviewed 24 transbronchial biopsies (TBBs) performed at the time of, or subsequent to, a clinical diagnosis of OB of chronic rejection in eight heart-lung and single-lung transplant recipients at the Methodist Hospital, Houston, Texas. These were compared with 36 randomly selected control TBBs from 14 patients without a clinical or histopathologic diagnosis of OB of chronic rejection at any time in their course. In the former group of TBBs, nine (38%) were diagnostic of OB and seven (29%) showed CIB suspicious for chronic rejection. TBBs diagnosed as negative had significantly fewer samples of tissue (1.0 versus > 3) and significantly smaller size of tissue (1.44 mm versus > 4.5 mm) than did TBBs in the OB or CIB categories. When strict criteria were used, no false diagnoses of OB of chronic rejection were made on control TBBs. However, OB associated with bronchiolitis obliterans organizing pneumonia of viral pneumonitis was present in three (8%) and CIB in 15 (42%) of control TBB associated with acute rejection and infection. This finding emphasizes the nonspecificity of CIB. We conclude that TBB is potentially useful in diagnosing OB of chronic rejection in some cases and in suggesting chronic rejection in other cases. Inadequate sampling by the bronchoscopist was the major reason for a negative biopsy.