Dendritic cells, macrophages and lymphocytes infiltrating the epithelia of sino-nasal lesions were investigated immunohistochemically using antibody S-100 protein for dendritic cells, Ki-M1P for macrophages, MT1 for T-cells and L26 for B-cells, respectively. Seventy-five specimens of sino-nasal mucosae involving 51 cases of metaplastic squamous epithelium (MSE), 12 of inverted papilloma, 1 of atypical hyperplasia and 11 of squamous cell carcinoma (SCC) were studied. Dendritic cells were observed in 12 cases of MSE (23.5%), in 4 of inverted papilloma (33.3%), and in 6 of SCC (54.5%). More dendritic cells were found infiltrating MSE of the infective and fibrotic types than of the mild and edematous types, histopathologically. Furthermore, infiltration correlated with the degree of squamous differentiation in MSE, but not in SCC. The number of macrophages was significantly smaller in MSE than in columnar ciliated epithelium, while that in SCC was significantly greater than that in MSE. With dendritic cell infiltration macrophages in MSE as well as SCC and T-cells in and around the carcinoma were often increased. These findings suggest that dendritic cells may be attracted to maturative squamous epithelium, and may play an important role in the immunologic defence mechanisms of MSE. Dendritic cells, in contrast, may play a role in the cellular immune response of T-cells against the carcinoma.