B-cell chronic lymphocytic leukaemia (B-CLL) is characterized by a high frequency of infections, including those of viral aetiology. Previous reports have demonstrated a specific immunologic response to influenza virus vaccine in B-CLL patients with normal IgG levels. In this study, we have evaluated different immunophenotypically defined B and T cell subsets in 18 B-CLL patients before immunization with killed-influenza-virus vaccine. A correlation between immunological response to vaccination and both absolute numbers of CD4+/CD45RA+ naive T cells and CD5- B cells was found. These data may suggest a supporting role of the CD4+/CD45RA+ T cell subset in the specific antibody response to vaccination with influenza virus vaccine in B-CLL patients.