It is possible to differentiate to some degree to bronchiolitis obliterans organizing pneumonia (BOOP) of unknown etiology from idiopathic usual interstitial pneumonia (UIP) on the basis of clinical symptoms, chest X-ray findings, pulmonary function test and blood gas findings, but in certain patients it can be difficult. Cell findings in bronchoalveolar lavage fluids (BALF) were compared between 8 patients with BOOP and 28 patients with UIP, both diagnosed histologically by open lung biopsy. Findings characteristic of BOOP included (1) an increase in the percentage of lymphocytes, (2) an increase in the percentage of eosinophils, and (3) a decrease in the OKT4+/OKT8+ ratio. These findings provide useful information in making a clinical diagnosis of BOOP and in evaluating proper steroid treatment in patients who unable to undergo open lung biopsy.