DiGeorge syndrome (DGS) is a congenital disorder characterized by facial dysmorphies, hypoparathyroidism, cardio-vascular malformations and variable T-cell deficiency due to thymic hypoplasia. We present a male infant with complete absence of T-cells and thymus successfully treated by repeat blood lymphocyte transfusions. According to previous experience of bone marrow transplantation in a similar patient, T-cell reconstitution is possible by peripheral expansion of mature donor cells, independent of stem cell engraftment. Our patient received at the age of four months an initial transfusion of blood lymphocytes from his HLA-identical brother, containing 1x108 T-cells/ kg. Ten days later he developed mild cutaneous GvHD, which resolved spontaneously without immunosuppressive therapy. Lymphocyte transfusions were repeated three times at 4 weeks intervals without reappearance of GvHD. T-cells developed within two months after the first transfusion and since then remain at levels between 500–1000/μl with a proportion of 25% disclosing a CD45RA+ phenotype. In chimerism analysis T-cells are exclusively of donor origin. The T-cell receptor repertoire is similarly diverse as in the donor. After more than 2 years of follow up the boy is not suffering from severe infections and shows no signs of autoimmune disease. Following immunization, antibody titres normalized. Conclusion: In infants with complete DGS transfusions of blood lymphocytes from a matched family donor may lead to long-lasting engraftment of functional T cells and stable chimerism and represents a potentially helpful therapeutic option.