Four patients had localized lesions of Woringer-Kolopp disease (pagetoid reticulosis). The clinical appearances and histopathologic features with light microscopy of the four cases were similar. However, electron microscopy in all four cases and enzyme histochemistry studies and immunophenotyping with monoclonal antibodies in three cases disclosed two subgroups of patients. The features of the predominant cells in the intraepidermal infiltrate of three cases were most consistent with those of helper T lymphocytes with cerebriform nuclei, as found in many cases of mycosis fungoides. In one case, however, the epidermis contained a predominant population of small, medium, and large lymphoid cells of cytotoxic/suppressor T-lymphocyte cell phenotype. It remains to be determined whether Woringer-Kolopp disease is a variant of mycosis fungoides (ie, within the spectrum of the cutaneous T-cell lymphomas), a benign reactive process, or a syndrome with various causes.