This flow cytometric study was designed to identify, characterize and quantify alveolar monocyte-like cells in healthy volunteers and in patients with chronic inflammatory lung disease. Cells were obtained by bronchoalveolar lavage (BAL) from 19 patients with sarcoidosis, 29 with idiopathic pulmonary fibrosis, 10 with extrinsic allergic alveolitis, 19 with collagen vascular disease, and from 10 healthy volunteers. By taking advantage of the distinct electro-optical features of alveolar macrophages (AMs) and monocyte-like cells, the numbers of alveolar monocyte-like cells were counted, the cell dimensions calculated, and the densities of antigens on the surface of alveolar monocyte-like cells and AMs were compared. By using a panel of monoclonal antibodies detecting CD11b, CD14, CD16, and human leucocyte antigen-DR (HLA-DR), the immunophenotypes of these cells were selectively characterized. In the BAL fluid of patients with chronic inflammatory lung disease, significantly increased numbers of alveolar monocyte-like cells were detected that exhibited an immunophenotype intermediate between blood monocytes and mature AMs. Positive correlations were found between numbers of monocyte-like cells and expression of the monocyte-associated surface antigens CD11b and CD14 on total AMs; in contrast, an inverse relationship existed between monocyte numbers and expression of the macrophage-associated surface antigens CD16 and HLA-DR. When the patients were assigned to two groups according to the percentage of BAL monocyte-like cells being lower or higher than 13% (= mean value of the controls +2SD), it could be demonstrated that a high percentage of BAL monocyte-like cells was associated with significantly reduced lung function parameters. In summary, our flow cytometric data strongly support the view that considerable numbers of blood monocytes are recruited to the bronchoalveolar space in patients with chronic inflammatory lung disease.