The aim of this study was to investigate the possibility of distinguishing between patients with similarities in clinical presentation, suffering from three frequently occurring interstitial lung diseases, by means of discriminant analysis, using a number of selected variables derived from bronchoalveolar lavage fluid (BALF) analysis. The study involved all 277 patients, who had an initial bronchoalveolar lavage (BAL) in the period 1980-1990. These patients belonged to the following diagnostic groups: sarcoidosis (n = 193), subacute extrinsic allergic alveolitis (EAA) (n = 39) and idiopathic pulmonary fibrosis (IPF) (n = 45). Thirty healthy volunteers were used as controls. Cellular and non-cellular constituents of BALF were evaluated. Variables, which could be used to discriminate among the three diagnostic groups were: yield of recovered BALF, total cell count, and percentages of alveolar macrophages, lymphocytes, polymorphonuclear neutrophils, eosinophils and plasma cells in BALF. When the set of data used to predict the membership of patients to diagnostic groups (test set) was the same as that in which the discriminant analysis was performed (learning set), 93% of the cases were correctly classified. This percentage decreased to 90%, however, when the test set was different from the learning set. It is possible to discriminate among patients with sarcoidosis, EAA or IPF with these selected variables. It appears that bronchoalveolar lavage (BAL) is useful as an adjunct in concert with other diagnostic methods.