We present the case of a 47-year-old patient who was seen for recurrent opportunistic infections. Immunophenotypic analyses disclosed severe reduction of CD4+ T cells. Repeated Elisa, Western blot and polymerase chain reaction tests for HIV were negative. The low CD4+ T lymphocyte count unaccompanied by increased CD8+ T lymphocytes and hypergammaglobulinemia, along with negativity for HIV infection, suggested the diagnosis of idiopathic CD4+ lymphocytopenia (ICL). The patient's clinical manifestations and laboratory results conformed with the case definition of ICL established in 1992 by the Centers for Disease Control of Atlanta, i.e., CD4+ T cells < 300/mm3 on two occasions and no evidence of HIV infection. In vitro analyses evidenced depressed lymphoproliferative responses to mitogens such as concanavalin A and pokeweed mitogen, while the expression of Fas antigen on peripheral lymphocytes and the percentage of apoptotic cells after propidium iodide staining were increased. Since in vitro concanavalin A stimulation inhibits T cell proliferation and induces apoptosis, these results suggest that the patient's lymphocytes are susceptible, in vivo, to an apoptotic signal.