Ultrastructural analysis of oral lichen planus was performed in 18 cases, focusing on cell-to-cell interactions. In the peripheral portion of the lesion, the most consistent findings were a widening of intercellular spaces, separation of the basement membrane (BM) from basal cells and scarce inflammatory cells. In the central portion of the lesions basal cells and BM showed severe damage and numerous inflammatory cells infiltrated into both the epithelium and subepithelial stroma. The infiltrates predominantly consisted of T-lymphocytes, a few Langerhans cells (LC) and macrophages. Most lymphocytes were large and positive for CD45RO. Dendritic LC and macrophages with cytoplasm containing abundant organelles were seen in the epithelium and subepithelial stroma, respectively. Close contacts of lymphocytes with LCs, macrophages and also with keratinocytes were sometimes observed in the central portion. In the subepithelial stroma, some lymphocytes contacted an HLA-DR+ dendritic cell, which was possibly a macrophage, forming a rosette-like arrangement. Conjugations between CD4 cells and dendritic cells (possibly LC) and also between CD8 cells and basal cells were observed in the epithelium. These T cells were large in size, and the CD8 cells which made contact with degenerated keratinocytes possessed cytoplasm containing numerous polarized organelles and a nucleus toward and contrary to the contact side, respectively. These lymphocytes expressed LFA-1 on the cell surface, and many basal cells exhibited ICAM-1. These findings indicate that T cells may receive information from LC and macrophages concerning degenerative keratinocytes, and that informed T cells attack perhaps the keratinocytes.