We report a 77-year-old woman with erythrosquamous plaques on the legs for 10 years. Biopsies taken 4 years ago revealed a pagetoid reticulosis with a massive epidermal cell infiltrate. She has now also developed a typical tumor of mycosis fungoides. The epidermal infiltrate of the two types of lesions bore the surface membrane marker for T lymphocytes rich in both T helper and T cytotoxic-suppressor subsets (ratio, 1.8). Both lesions also showed dendritic OKT6-positive Langerhans cells, and staining of the intercellular material with the DR locus of human lymphocyte antigen was positive. A dense dermal infiltrate was evident only in the mycosis fungoides lesions, and it was similar to that in the epidermis. Electron microscopy of the two lesions showed the atypical cells described in mycosis fungoides and the Sézary syndrome. These observations suggest that disseminated pagetoid reticulosis probably is a variant of mycosis fungoides.