Study Objective: The purposes of this study were as follows: (1) to establish the positivity rate and complication rate of transbronchial lung biopsies in the treatment of lung transplant recipients; (2) to determine the sensitivity of transbronchial lung biopsy specimens for the diagnosis of clinically suspected acute rejection and cytomegalovirus pneumonia; and (3) to examine the results of surveillance transbronchial lung biopsies in clinically and physiologically stable recipients. Design: Retrospective review and analysis of 203 consecutive procedures. Setting: Washington University Lung Transplantation Program, Washington University School of Medicine and Barnes Hospital, St. Louis, Mo. Patients: Fifty-five lung transplant recipients. Interventions: Biopsies were done with 2-mm fenestrated forceps using fluoroscopic guidance. Two hundred three bronchoscopies with transbronchial lung biopsy were performed for clinical indications (n = 88), routine surveillance (n = 90), or follow-up of a previous biopsy (n = 25). Biopsy specimens showing acute allograft rejection were classified according to the scheme recommended by the Lung Rejection Study Group. Measurements and Results: The positivity rate and complication rate were determined for the procedures. In procedures performed for clinical indications, the sensitivity for the diagnosis of acute rejection and cytomegalovirus pneumonia was calculated by a decision-to-treat analysis. A specific histologic diagnosis was detected in 69 percent of the clinical procedures, 57 percent of the surveillance procedures, and 64 percent of the follow-up procedures. For clinical indications, the sensitivity of transbronchial lung biopsy was 72 percent for the diagnosis of acute rejection and 91 percent for the diagnosis of cytomegalovirus pneumonia. Surveillance biopsy specimens often showed clinically inapparent rejection or cytomegalovirus pneumonia. The overall complication rate was 8.9 percent; none of the complications were life threatening. Conclusions: Transbronchial lung biopsy is a useful and safe procedure in the treatment of lung transplant recipients. When performed for clinical indications, the procedure proved to be sensitive for the diagnosis of acute rejection and cytomegalovirus pneumonia. When performed for surveillance in clinically and physiologically stable recipients, the incidence of rejection and cytomegalovirus pneumonia was unexpectedly high; the potential clinical implications of these findings will require further study.