Febrile ulceronecrotic Mucha-Habermann disease (FUMHD) is a rare and severe variant of pityriasis lichenoides et varioliformis acuta. It usually presents erythematous macules, papules and ulceronecrotic plaques on the whole body, high fever and systemic manifestations. Methotrexate is used to treat FUMHD and known to be related to development of lymphoproliferative disorders. Herein, we report a case of development of primary cutaneous γδ T-cell lymphoma during treatment of methotrexate in a patient with FUMHD. A 74-year-old man presented erythematous macules and ulceronecrotic plaques on the whole body since 3 years ago. Histopathologic findings on the ulcer margin and erythematous macule showed parakeratosis, necrotic keratinocytes, subepidermal separation and perivascular lymphocytic infiltration. Diagnosed as FUMHD, he was started with steroid, doxycycline, dapsone and low-dose methotrexate for 3 months (7.5mg/wk for 1 month, 15mg/wk for 1 month). At that point, protruding masses on the both lower legs developed. Histopathologic results on the protruding masses were primary cutaneous γδ T-cell lymphoma with TCR gamma rearrangement positive and immunohistochemical stain results of CD3 positive, CD8 dominant pattern, Ki-67LI 80% and granzyme B positive. His skin lesions almost improved after 3 cycles of CHOP regimen, but chemotherapy made his general condition worsened and he expired due to sepsis.