Summary Secukinumab is an interleukin (IL)‐17 monoclonal antibody inhibiting T‐helper (Th)1‐mediated immune response. It has proven high efficacy for moderate to severe psoriasis but data on its long‐term toxicities are limited. We describe two patients who received secukinumab for clinically presumed psoriasis, but were subsequently diagnosed with mycosis fungoides (MF) following skin biopsies triggered by skin deterioration while on secukinumab. Previous studies suggested decreased numbers of regulatory T cells (Tregs) with increasing stage of MF, which may lead to the shift in the Treg/Th17 balance towards the Th17 pathway. Theoretically, the use of IL‐17 monoclonal antibodies to inhibit Th17 pathway may lead to further immunosuppression and disease progression in cutaneous T‐cell lymphoma (CTCL) by shifting the balance towards Tregs, although this hypothesis has not been proven. With uncertainty over the role of IL‐17 and Treg/Th17 as well as diagnostic challenges in CTCL, we recommend that patients should have a confirmatory skin biopsy prior to the commencement of biologic therapy.

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PCALCL is characterized by variable histopathological presentations and a broad cytomorphologic spectrum.