We have investigated immunohistologically the cutaneous immune infiltrate in the lesions of five patients with severe, extensive lichen planus of recent onset before and after 15 days of oral, low-dose cyclosporine therapy (3 mg/kg/day). Before therapy, we observed an abnormal bandlike cellular infiltrate localized in the papillary dermis, composed mostly of CD3+ cells, with a prevalence of CD4+ cells. Infiltrating lymphocytes showed markers of activation (HLA-DR antigens and interleukin 2 receptor), and there were many Langerhans (CD1+) cells in the dermal infiltrate. After 15 days of cyclosporine therapy, we observed a dramatic decrease in the total number of T cells and a corresponding decrease in interleukin 2 receptor-positive activated CD25+ cells and in antigen-presenting cells (CD1+ and CD14b+). These changes were concurrent with clinical improvement. Our results are compatible with the hypothesis that the inhibition of CD4 T cells by cyclosporine might explain the drug's therapeutic action and that the interaction between antigen-presenting cells and CD4 T cells is important in the pathogenesis of lichen planus.