Histopathologic and ultrastructural observations in a case of Pagetoid reticulosis (Woringer and Kolopp disease) are reported. The most important clinical feature of this disease is manifestation as a single very slowly enlarging verrucoid, plaque-type skin lesion without internal organ involvement. Histologically, the epidermis is extremely acanthotic and densely infiltrated with numerous atypical appearing cells that are entirely absent from the dermis, which simply contains a banal dense chronic inflammatory infiltrate. Ultrastructural study showed that there are two major types of unusual cells infiltrating the epidermis. One type is a large lymphoid cell similar to the stimulated lymphocyte. The second type differs from the first in having paler staining of the nucleus and cytoplasm and showing cell membrane damage. These cells probably derive by degeneration from the large lymphoid cells. The presence of transitional forms favors this interpretation. Occasionally, degeneration of these cells proceeds to their complete necrosis.