Lesional skin specimens from twenty-eight patients with mycosis fungoides were studied by evaluating immunohistochemical criteria, primarily with monoclonal antibodies. It was demonstrated that significant differences exist between the control and the premycotic-stage group in regard to the monoclonal antibodies BE1, BE2, and OKT9. The detection of specific antigenic determinants on the surface of cell populations early in the course of the disease seems to be of considerable value for the early diagnosis of the disease. Statistically significant differences were found between the premycotic stage and the plaque stage in regard to T lymphocytes, macrophages, OKT6 +, OKT4 +, OKT8 +, and BE2 + cells in the dermal infiltrate. Significant differences were also shown between the plaque and tumorous groups, concerning macrophages, T cells, and OKT9 + cells in the dermis, as well as epidermal dendritic cells. Differences between stages may supplement histologic data for the follow-up of the disease with or without treatment.