Proliferation of type II pneumocytes may be the most sensitive pathological indicator of alveolitis, and the density of type II pneumocytes reflecting the severity of the recent alveolar damage. Twenty-nine patients were divided into three groups by the severity of ground-glass opacities on the CT scans; the severe (acute), the moderate (subacute) and the mild (chronic) groups. We compared the density of type II pneumocytes in the transbronchial lung biopsy specimens and cell analysis of bronchoalveolar lavage (BAL) fluid with the ground-glass opacities. Clinical and laboratory findings and BAL fluid analysis also corresponded well with this grouping. Type II pneumocytes were selectively stained with an antibody against Thomsen-Friedenreich and the density of the type II pneumocytes was expressed as the number per 1 mm alveolar septal length. The densities of the type II pneumocytes in the severe, the moderate, the mild, and the control groups were 21.2+/-1.1, 14.4+/-7.2, 11.0+/-3.3 and 7.5+/-0.9/mm, respectively. There were significant differences between the acute group and the other three groups, and between the control and chronic groups. The density of type II pneumocytes is a useful index for evaluating alveolar damage even in mild alveolitis.