The purpose of this study was to assess the role of bronchoalveolar lavage (BAL) in differentiating usual interstitial pneumonia (UIP) from non-specific interstitial pneumonia (NSIP) and in predicting the prognosis in fibrotic idiopathic interstitial pneumonia (IIP). A retrospective review of 122 patients (age 58+/-8 years, 70 male) with UIP (n=87) and NSIP (n=35) was carried out. Prior to surgical lung biopsy, all of them underwent BAL and high-resolution-computed tomography (HRCT) of the chest. Neutrophil count in BAL fluid was higher in UIP (7.0%) than NSIP (3.0%) (P=0.027). In contrast, BAL lymphocyte count was significantly higher in NSIP (29.0%) than UIP (5.5%) (P<0.0001). In 62 patients whose HRCT findings were atypical for UIP, BAL lymphocytosis was more frequently observed in NSIP (20/33) than UIP (4/29) (P<0.001) and the absence of BAL lymphocytosis suggested a diagnosis of UIP rather than NSIP (odds ratio 12.7, P<0.001). Pathologic diagnosis of NSIP was the only independent factor predicting a longer survival of our patients (median follow-up 21 months) (hazard ratio (HR) 0.035, P=0.005). When NSIP was not included in the survival analysis, higher BAL lymphocyte count was the only independent predictor of a longer survival (HR 0.909, P=0.029). BAL is an useful non-invasive tool in fibrotic IIP, not only for excluding a variety of specific non-IIP diseases but also for narrowing the differential diagnosis and predicting the prognosis in the absence of the histopathologic diagnosis.