Idiopathic CD4⁺ T-cell lymphocytopenia (ICL) is a rare acquired T-cell immunodeficiency of unknown pathogenic basis. Six adults with ICL who developed opportunistic infections were investigated using extensive immunophenotyping analysis and functional evaluation of the chemokine receptor CXCR4. For all 6 patients studied, a profound defect in CXCR4 expression was detected at the surface of CD4⁺ T lymphocytes, in association with an abnormal intracellular accumulation of CXCR4 and of its natural ligand, the chemokine CXCL12. For all patients studied, CD4⁺ T-cell chemotactic response toward CXCL12 was decreased, whereas sensitivity to CXCL8 was preserved. CXCR4 recovery after ligand-induced endocytosis was impaired in ICL CD4⁺ T cells. Upon in vitro addition of interleukin-2 (IL-2), membrane expression of CXCR4 returned to normal levels in 5 of 6 patients, whereas intracellular accumulation of CXCR4 and CXCL12 disappeared. Upon therapeutic administration of IL-2, CD4⁺ T-cell count and membrane CXCR4 expression and function improved over time in 3 of 4 patients treated. Therefore, our data indicate that ICL is associated with defective surface expression of CXCR4, which may be reversed by IL-2.