Large numbers of CD30-positive tumor cells are not typically observed in mycosis fungoides (MF), but CD30 expression may occur on large cells of MF that have transformed into high-grade large cell lymphoma. Of 202 patch/plaque phase MF cases studied by immunohistochemistry, we encountered 4 patients with patch-phase MF at stage Ia or Ib, whose lesions contained a high proportion (greater than 50%) of CD30-positive tumor cells within the epidermis. The morphologic and immunohistochemical features of these four cases were otherwise similar to those of other patch-phase MF cases, and were different from those of pagetoid reticulosis. More importantly, the clinical behavior of these cases did not differ from that of other cases of early MF without CD30 expression. The mechanism underlying the high levels of CD30 expression by epidermotropic tumor cells is unknown. With increased use of the CD30 immunohistochemical stain in the diagnosis of cutaneous lymphomas, similar cases are likely to be encountered, and perhaps will be evaluated for their clinical behavior.