Chronic rejection after lung transplantation, manifesting as bronchiolitis obliterans syndrome (BOS), has become the dominant challenge to long-term patient and graft survival. In order to elucidate risk factors for development of BOS we utilized the 1995 revision of the working formulation for the classification of lung allograft rejection (), and devised a quantitative method to retrospectively study lung transplant biopsies from all patients who survived at least 90 d. All transbronchial biopsies were regraded 0 to 4 for acute perivascular rejection and lymphocytic bronchitis/bronchiolitis (LBB), and the grades were totaled over a period of time to give two scores, respectively, for each patient. Also examined were timing of acute rejection and LBB episodes and decreased immunosuppression defined as two or more cyclosporine A levels < 200 ng/ml. Sixty-six patients with BOS and 68 with no BOS (NBOS) satisfied our criteria for inclusion in the study. Demographics including age, sex, and primary diagnoses were similar. The mean perivascular score for BOS was 6.2 over a mean follow-up of 822 d (range, 113 to 2,146) compared with 3.2 for NBOS over 550 d (range, 97 to 1,734) mean follow-up. Airway scores were 5.3 and 1.7, respectively, for the same follow-up periods. There was no correlation between length of follow-up and rejection or LBB scores, although mean length of follow-up for the two groups was significantly different. Late acute rejection and LBB were significantly associated with BOS as was decreased immunosuppression. In addition to perivascular rejection, LBB, late acute rejection, and decreased immunosuppression are significant risk factors for the development of BOS. Analysis of the current data leads us to believe that LBB, in the absence of infection, is in fact a manifestation of acute rejection, with similar implications for graft function as acute perivascular rejection.