Erythema dyschromicum perstans (EDP) and vitiligo are two cutaneous pigmentary dermatoses of unknown etiology. In the present study, the leukocyte infiltrates in the affected skin of EDP and vitiligo patients were studied using the avidin-biotin (ABC) immunoperoxidase technique and monoclonal antibodies which recognise the following mononuclear cell subgroups: T-suppressor/cytotoxic (CD8-Leu-2), T-helper (CD4 = OKT4), T-suppressor + macrophages (Leu-15), Pan T (CD3 = Leu-4), macrophages (Leu-M3) and Langerhans cells (CD1 = Leu-6), and other cellular markers such as Ia antigens and the Interleukin-2 receptor (CD25 = TAC). The immunocytochemical analysis showed a selective accumulation of CD3+, CD8+, Leu-15-, T-cytotoxic cells in the epidermis of both EDP and early lesions of vitiligo. In addition, an increase in the number of epidermal Langerhans cells (CD1+) was observed in some cases of EDP and vitiligo. The CD4/CD8 ratios in affected and uninvolved skin for both disorders were not significantly different, although values lower than unity were only observed in the infiltrates of affected skin. Ia antigen positivity was observed in the dendritic cells of the dermis and epidermis, as well as in most of the lymphoid cells within the infiltrates for both diseases. Macrophages (Leu-M3) in EDP dermal infiltrates were generally found adjacent to extracellular melanin pigment. Lymphocytes expressing TAC (CD25) surface antigens were also present in the dermal infiltrates. These morphological observations suggest a possible immune cell participation in the dyschromia of such cutaneous disorders.