A patient with Sézary's syndrome is described. Surface marker analysis of her peripheral blood lymphocytes exhibit a phenotype characteristic of mature helper T cells (OKT3+, OKT4+, OKT8-, OKT6-). Serological studies revealed a polyclonal hyperimmunoglobulinemia with large amounts of IgE and IgA and circulating immune complexes that activate the complementary system via alternative pathway. The patient's cells showed a helper activity on normal B cell differentiation after a 7-day co-culture with pokeweed mitogen. The relevance of this helper phenotype and the function on in vivo polyclonal B cell activation is discussed.