Infants with complete DiGeorge syndrome have thymic aplasia with no measurable T cell function. This report describes restoration of T cell function and TCR repertoire during a 30 month period after thymic transplantation in a patient with complete DiGeorge syndrome. T cell function developed beginning at 7 months after transplantation. Currently, peripheral blood mononuclear cell (PBMNC) responses to PHA, ConA, and PWM, are normal. The patient's PBMNC displayed normal reactivity in mixed lymphocyte cultures but were tolerant of the thymic. TCR V repertoire studies (by flow cytometry and RT PCR and sequencing) showed an early clonal expansion of T cells and a later normalization of TCR repertoire. The patient developed normal serum immunoglobulin levels and normal antibody responses to tetanus toxoid and pneumococcus serotype 3. The patient is clinically well on supplemental calcium. Thymic transplantation should be considered in patients with complete DiGeorge syndrome.