In bronchoalveolar lavage fluid (BAL) from nine patients with alveolar proteinosis (AP), total and differential cell count and T-lymphocyte phenotyping were done and compared with those in 12 healthy volunteers comparable as to age and tobacco consumption. Although total cell count was not significantly different from that in control subjects, the most prominent feature in patients was an increase in the number of CD4 and CD8 T-lymphocytes within the alveoli. Conversely, the macrophage population was significantly reduced. The ratio of CD4/CD8 T-lymphocytes tended to be high, but this increase did not reach statistical significance. The pathophysiologic mechanism and the meaning of these alveolar cell changes in AP remain to be elucidated.