The precise immunopathological mechanism of extrinsic allergic alveolitis explaining the clinical picture as well as the pathological findings is not known. Bronchoalveolar lavage can be a diagnostic help and a method to unravel the pathophysiology of this disease. In the acute stage of extrinsic allergic alveolitis or within 24 h after antigen exposure an increase in the number of neutrophils is seen. After the acute stage, the number of lymphocytes is even higher than in sarcoidosis. In extrinsic allergic alveolitis as well as in sarcoidosis these lymphocytes are mainly T lymphocytes. However, the distribution of OKT 4 and OKT 8 positive lymphocytes was clearly different in both diseases. In sarcoidosis OKT 4 lymphocytes predominate (OKT 4/8 = 7.8) while in extrinsic allergic alveolitis an increase of both OKT 4 and OKT 8 lymphocytes has been found (OKT 4/8 = 1.5). Whether a type III Arthus reaction or a type IV delayed hypersensitivity with an early component is involved, is discussed.