Cyclic neutropenia is a rare hematological disorder characterized by periodical severe granulocytopenia. A stem cell defect and/or immunological abnormalities are considered to play a role in this disease. Here we describe the case of an adult woman who was diagnosed as having both cyclic neutropenia and severe hypogammaglobulinemia. Her clinical history revealed that one or both abnormalities had been present since childhood. Normal in vitro growth of the patient's bone marrow CFU-GM was observed, while immunological analysis revealed the presence of a persistent excess of activated (HLA-DR+) CD8+ T lymphocytes in both bone marrow and peripheral blood. These T lymphocytes have been shown to be polyclonal by DNA analysis, and their role in determining the clinical picture of our patient remain uncertain since they could not be shown to produce inhibitors of in vitro CFU-GM growth. Intermittent low doses of human recombinant G-CSF were able to improve neutropenia and completely prevent infectious symptoms, thus confirming the efficacy of this cytokine in cyclic neutropenia patients.