Large-cell transformation of mycosis fungoides (LCTMF) is rare, histologically distinct, with an aggressive clinical course; yet is not recognised as an independent entity in classification systems nor in staging systems for mycosis fungoides and Sézary syndrome (MF/SS). Herein, the patterns of care and survival outcomes for patients with LCTMF are described, with prognosis compared to published data of non-transformed MF/SS. Eligibility required clinicopathological diagnosis of LCTMF (1/1/1990-31/10/2021), managed at Peter MacCallum Cancer Centre. Eighty-three patients were eligible. Median follow-up was 8.0 years. At the time of LCTMF, 36% had early-stage MF (IA-IIA), 76% had cutaneous-only LCTMF. The most common first-line treatments were localised radiotherapy (48%) and multiagent chemotherapy (23%). Median overall survival (OS) from LCTMF diagnosis was 3.5 years (95% [confidence interval] CI: 2.2-8.2). Three prognostic groups of LCTMF were identified: unifocal cutaneous only, multifocal cutaneous only and extracutaneous (median OS: 4.6, 2.5 and 1.1 years, respectively; p = 0.005). Unfavourable prognostic factors were advanced age and extracutaneous LCTMF. In conclusion, treatment pathways for patients with LCTMF were varied, and prognosis was poor, despite >1/3 having early-stage MF. However, differences in prognosis were suggested, with unifocal cutaneous LCTMF associated with greater OS. Given prognostic differences from MF/SS, consideration to include LCTMF in staging systems is warranted.