Background: Mycosis fungoides (MF) is a neoplasm of skin-homing CD4(+) helper T (TH) lymphocytes with dysregulation of TH1 and TH2 immunity. Diagnosis of MF is challenging, as there is significant morphologic overlap with other dermatologic entities.

Objective: We investigated diagnostic utility of TH1- and TH2-specific markers, T-bet, and GATA-3, respectively, in MF and its reactive and neoplastic mimics.

Methods: Immunohistochemical staining for CD3/T-bet and CD3/GATA-3 was performed on inflammatory dermatoses (n = 56), MF (n = 37), Sezary syndrome (SS; n = 8), and cutaneous anaplastic large cell lymphoma (C-ALCL; n = 14).

Results: Inflammatory dermatoses showed epidermal T cells predominantly expressing GATA-3, except psoriasis, which exhibited a mixed GATA-3/T-bet staining. In contrast, neoplastic T cells in patch stage MF showed markedly increased T-bet positivity with minimal GATA-3 expression. Plaque stage MF had a mixed T-bet/GATA-3 phenotype, whereas tumor stage MF and SS exhibited diffuse GATA-3 expression. C-ALCL lacked significant staining for both markers.

Conclusions: Sample size was relatively small. Conclusions: A predominance of T-bet(+) T cells in the epidermis support patch stage MF over dermatitis. A predominance of GATA-3(+) T cells in the dermis support CD30(+) MF with large cell transformation over C-ALCL. These stains do not allow distinction between dermatitis and cutaneous infiltrates of SS.