Background: Sézary syndrome (SS) is a rare and aggressive type of cutaneous T cell lymphoma characterized by an intensely pruritic, exfoliative rash, known as erythroderma, with cutaneous and systemic dissemination of clonal CD4+ T cells into the blood and lymph nodes. We report through a series of 4 cases, the experience of the hematology laboratory in the diagnosis of the syndrome of Sézary. Methods: Four patients with SS were identified retrospectively among patients with cutaneous T-cell lymphoma followed up in the dermatology department and diagnosed in the hematology laboratory of the Avicenne military hospital in Marrakesh. Results: Four patients with SS were described: three men and one woman, mean age at diagnosis 62 years (55-71). All the patients showed generalized dry erythroderma, pruritus and lymphadenopathy. Palmo-plantar hyperkeratosis, nail lesions and alopecia were also present. The white blood cell count was elevated (>10,000 WBC/ mm3) in all 4 patients with a mean value of 17,276 and one patient among these showed an elevation of eosinophils (> 500/ µl). The blood smear showed the presence of 65% of small to medium-sized cells with a high nucleocytoplasmic ratio and cerebriform nuclei typical of Sézary cells and suggests the diagnosis of SS. Conclusion: Sézary syndrome is a rare subtype of cutaneous T-cell lymphoma characterized by erythroderma, circulating neoplastic T cells, and poor prognosis. Microscopic findings must be correlated with the clinical presentation to make the diagnosis.