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. Now the inventors showed the expression of CD38 by Sézary cells and in CD4+ blood cells of patients with Sezary syndrome. CD38 therefore appears as a useful diagnostic, prognostic and follow-up marker, and as a potential therapeutic target in T-cell lymphomas. Therapeutic depletion of CD38-expressing cancer cells would eliminate tumor cells.