Early cutaneous T cell lymphoma clinically and histologically resembles benign inflammatory skin diseases, which sometimes makes it difficult to reach a correct diagnosis. It is recently reported that thymocyte selection-associated high mobility group box factor (TOX) serves as a molecular marker for histological diagnosis of early-stage mycosis fungoides (MF). To examine whether TOX could be a marker of tumour cells in different types of cutaneous lymphoma, we investigated immunohistochemical staining for TOX with the lesional skin of patch, plaque, and tumour MF, Sézary syndrome (SS), lymphomatoid papulosis (LyP), primary cutaneous anaplastic large cell lymphoma (PCALCL), adult T cell leukemia/lymphoma (ATLL), peripheral T-cell lymphoma, not otherwise specified (PTCL, NOS), atopic dermatitis (AD), and normal skin. TOX and CCR4 messenger RNA (mRNA) levels in lesional skin of MF/SS were also examined. Immunohistological staining showed that a high specific nuclear staining of TOX was observed at a high frequency in MF, SS, and PTCL, NOS. Tumour cells in LyP, PCALCL, and ATLL showed a slightly dim nuclear staining of TOX. TOX(+) cells in MF and LyP expressed surface molecules characteristics of tumour cells in these diseases. Lesional skin of SS expressed higher levels of TOX mRNA, compared to normal skin or MF lesional skin. Moreover, TOX expression significantly correlated with CCR4 expression. TOX may be a specific marker for tumour cells in some types of cutaneous lymphoma.