Thymus transplantation is an experimental procedure in which allogeneic, cultured, postnatal thymus tissue is transplanted into an athymic infant. Almost all transplant recipients are infants with complete DiGeorge anomaly. The donor thymus is obtained as discarded tissue from pediatric heart surgery after informed consent. It is sectioned, and maintained in culture while donor screening is performed. After 2-3 weeks in culture, the thymus tissue is transplanted into the quadriceps muscle of the athymic infant under general anesthesia in the operating room. Temporary immunosuppression is required in approximately half of transplant recipients. In a series of 67 consecutive patients, the 1-year survival rate was 76%. The median age of survivors is 7.8 years. Naïve CD4+ and CD8+ T cells develop over the first 6-9 months. Immunosuppression, immunoglobulin replacement therapy, and Pneumocystis prophylaxis are discontinued at approximately 1 year post-transplantation.