Analysis of bronchoalveolar lavage cellular constituents has been recommended as a valuable method for the characterization of the inflammatory cellular population and for studying cellular interactions in the lower respiratory tract of patients with idiopathic pulmonary fibrosis (IPF). However, the clinical relevance of the enumeration of cells in bronchoalveolar lavage fluid (BALF) from patients with IPF remains controversial. We therefore examined the correlations between BALF cellular constituents and both the histopathologic abnormalities and the subsequent clinical response to corticosteroid therapy in 26 newly diagnosed, untreated patients with IPF. The BALF lymphocytosis was associated with moderate-to-severe alveolar septal inflammation (p less than 0.0005) and with a relative lack of histologic honeycombing (p less than 0.05). On the other hand, BALF neutrophil and eosinophil contents did not significantly correlate with any of eleven particular histopathologic abnormalities, and BALF neutrophil and lymphocyte contents did not correlate with the degree of clinical impairment (quantitated by a composite score based on dyspnea, radiographic abnormalities, and physiologic impairment) upon presentation. However, BALF eosinophil content correlated significantly with the severity of clinical impairment, higher eosinophil counts being associated with more severe initial clinical impairment (p less than 0.01). Neither pretreatment BALF neutrophil nor eosinophil content was related to the frequency or magnitude of subsequent clinical change in 20 patients evaluated before and after 1 yr of corticosteroid therapy. In contrast, pretreatment BALF lymphocytosis was associated with significant subsequent clinical improvement (p less than 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)