Central nervous system (CNS) involvement in mycosis fungoides (MF) is a rare occurrence with devastating consequences. Large-cell transformation (LCT) of MF is associated with a poor prognosis and increased extracutaneous manifestations. In this report, we present 5 patients with MF, LCT, and CNS disease to better define their presentation, treatment, and prognosis and present critical differences they have compared with their nontransformed counterparts. We found that patients with LCT and MF are at increased risk of CNS involvement. In addition, whereas patients with nontransformed MF typically have significant skin and visceral involvement at the time of CNS disease, some patients with LCT have no extracutaneous symptoms and their skin MF is in remission at the time of CNS presentation. Transformed patients tend to present with speech and visual difficulties secondary to mass lesions. Therefore, we recommend at least annual screening of patients with transformed MF for CNS disease with head computerized tomography with orbital cuts.