In order to evaluate the diagnostic values of bronchoalveolar lavage (BAL) and transbronchial lung biopsy (TBLB) in interstitial lung diseases, we made a retrospective study of a total of 43 patients all of whom underwent examinations for both conditions at the same time. In the BAL examination, fractional analyses of cell differential counts were applied to the first 30 ml lavage (FBAL-I, bronchial lavage) and to the following 50 ml aliquot of the second and third lavages (FBAL-II and -III, the latter supposedly alveolar lavage) lavages respectively. This technique revealed the unique characteristics of bronchial and alveolar inflammations in a separate data and this is assumed to be a precise reflection of the pathogenesis of interstitial lung diseases. The evaluation values (EV) were graded from 1 to 5 for both results according to the following diagnostic significance: EV1: not useful, EV2: normal or only slight changes from normal level, EV3: moderate changes although not diagnostic, EV4: quite compatible findings for diagnosis and EV5: definitely diagnostic. The 19 patients (43%) whose EV in TBLB were either 4 or 5 were diagnosed as having sarcoidosis (3), hypersensitivity pneumonitis (1), tuberculosis (2), pneumoconiosis (6), histiocytosis X (3), pulmonary infiltration with eosinophilia syndrome (PIE) (1), collagen vascular diseases (1), panbronchiolitis (1) or lymphagiomyomatosis (1). Thirteen of them had a BAL EV 4 which consisted of rather characteristic patterns of cell differential counts: thus their diagnoses were reconfirmed. In the other 4 patients with hypersensitivity pneumonitis, tuberculosis or PIE, the diagnoses were established by the findings of BAL EV 4 although the EV of TBLB were 3 or 2. These results indicate that BAL alone hardly establishes a diagnosis but it can support the TBLB findings and increase a diagnostic ability by 10% in total. Basophilic leukocytes, mast cells and Langerhans cells in BAL were rather non-specific, but the former two frequently appeared in allergic states and the latter two were often present in fibrotic lungs. Histiocytosis X is indicated if Langerhans cells are recovered from more than a small percentage of the total cell counts.