The development of airflow obstruction, most often due to bronchiolitis, is a significant cause of morbidity and mortality in recipients of allogeneic BMT. Current consensus holds that this airways disease is the result of chronic GVHD and/or CMV infection. However, recent studies of idiopathic forms of BRO have demonstrated a striking influx of neutrophils into the lungs of affected individuals. Reasoning that the immune cell populations involved in tissue injury associated with either CGVHD or CMV infection would consist predominantly of lymphocytes, we tested this hypothesis by performing BAL in 12 adults with minimal or absent smoking histories who developed significant airflow obstruction (FEV1/FVC = 80.7 +/- 1 percent preBMT and 56.8 +/- 2.4 percent postBMT; p less than 0.001) following allogeneic BMT. Eleven of 12 patients had evidence of chronic, stable GVHD at the time of the study. In contrast to non-BMT patients with BRO, BAL defined two distinct patterns of lung inflammation in the BMT patients with airflow obstruction: (a) neutrophil predominance (five patients; neutrophil percentage = 20.2 +/- 6.6 percent); and (b) lymphocyte predominance (three patients; lymphocyte percentage = 35.9 +/- 12.1 percent). These data suggest that the pattern of inflammation in the lungs of BMT patients with BRO is not uniform and is not associated with active microbial infection. From these results, it is inferred that the airways injury in BMT patients may reflect diverse pathogenetic mechanisms initiated in the context of CGVHD and cytotoxic drug therapy.