Mycosis fungoides (MF), the most common type of primary cutaneous T-cell lymphoma, undergoes large cell transformation in only a minority of cases that present in patch stage. We describe a 52-year-old patient with a history of unilesional patch stage MF treated with localized radiotherapy. He had disease recurrence 5 years later and began psoralen plus ultraviolet A treatment with good response. After inguinal lymphadenopathy was detected on physical examination, a follow-up PET scan showed a fluorodeoxyglucose-avid inguinal node, which was evaluated by fine needle aspiration. Cytomorphology showed numerous Reed-Sternberg-like cells, highly suspicious for Hodgkin lymphoma. Histologic sections revealed isolated sinusoidal involvement by pleomorphic CD30-positive tumor cells, diagnostic of large cell transformation of MF in light of clinical history. The cells were also positive for CD15 and CD4. This case emphasizes morphologic features of an entity with limited description in the cytology literature and adds to the growing number of reported CD15-positive T-cell lymphomas.