T-cell lymphomas are a heterogeneous group of malignancies involving T lymphocytes and generally characterized by a poor prognosis. Among them, cutaneous T-cell lymphomas involve primarily the skin. Mycosis fungoides and Sézary syndrome are the most frequent cutaneous T-cell lymphomas. The Inventors showed that malignant T cells express CD33 in patients with Sézary syndrome, mycosis fungoides and Hepatosplenic T-cell lymphoma (HSTL). CD33 therefore appears as a useful diagnostic, prognostic and follow-up marker, and as a potential therapeutic target in T-cell lymphomas.