Sézary syndrome (SS), a type of cutaneous T-cell lymphoma with a poor prognosis, is characterized by erythroderma and leukemic involvement. Because of the rarity of SS and difficulty in diagnosis, data on this aggressive malignancy are scarce. In this review, the diagnosis and pathology of SS are summarized and an update is provided, highlighting microscopic features and novel molecular findings. The diagnostic challenge of SS is described, with an emphasis on the differential diagnosis of erythroderma and key points in distinguishing SS from other cutaneous T-cell malignancies. Finally, the prognosis is discussed, focusing on large, recent studies of SS patients.