Atypical intraepidermal T cells in Pagetoid reticulosis did not express CD45RO, the low molecular isoform of CD45. These cells did express other CD45 isoforms in the disseminated aggressive variant (Ketron-Goodman type). The complete CD45 family was lost from the cell surface in the benign localized variant (Woringer-Kolopp type). Because the presence of CD45 is crucial for the function of the lymphocyte-specific tyrosine kinase p56lck, which is associated with lymphocyte growth and transformation, this may be the molecular basis for the non-aggressive nature of this clonal T-cell proliferation.