An eosinophilia of 3% or more was present in 13.3% of a series of 1,084 broncho-alveolar lavages done in our department. An eosinophilia of less than 10% was of no diagnostic value. Idiopathic diffuse interstitial fibrosis, hypersensitivity pneumonitis, and asthma were the principal diagnoses encountered in cases with an eosinophilia of between 10 and 40%. An eosinophilia of greater than 40% was mainly observed in cases of chronic idiopathic eosinophilic pneumonia. A linear correlation (p less than 0.01; r = 0.545) was found between the blood eosinophils and that of the broncho-alveolar lavage; the existence of isolated cases where these two were dissociated raised the possibility of chemotactic factors acting independently on the blood and alveolar compartments.