Using immunohistologic methods, we studied the expression of the T-cell receptor (TCR)-associated antigens CD3, TCR-beta, and TCR-delta by cutaneous T cells in mycosis fungoides (MF) (36 patients) and a variety of inflammatory diseases (16 patients). Most T cells in the inflammatory diseases and patch/plaque mycosis fungoides expressed the immunophenotype characteristic of the vast majority of mature peripheral T cells: CD3+ TCR-beta+ TCR-delta-. In contrast, abnormal CD3/TCR-beta antigen expression was seen in 3 of 6 cases (50%) of tumor stage mycosis fungoides. Furthermore, we were able to document its evolution from the normal pattern present in earlier patch/plaque lesions of the two cases in which serial biopsies were available for study. Divergence of epidermal versus dermal CD3/TCR-beta antigen expression was seen in 2 of 34 (6%) of biopsies of patch/plaque mycosis fungoides but not in inflammatory controls. The TCR-delta+ cells were generally rare regardless of diagnosis. We conclude that inflammatory skin diseases and most patch/plaque mycosis fungoides are typically composed of T lymphocytes that resemble mature peripheral T cells in regard to their expression of TCR-associated antigens. In contrast, aberrant patterns of TCR-associated antigen expression can be seen in tumor stage MF, and, more rarely in patch/plaque MF.