This study will monitor the long-term effects of gene therapy in patients with severe combined immunodeficiency disease (SCID) due to a deficiency in an enzyme called adenosine deaminase (ADA). It will also follow the course of disease in children who are not receiving gene therapy, but may have received enzyme replacement therapy with the drug PEG-ADA.

ADA is essential for the growth and proper functioning of infection-fighting white blood cells called T and B lymphocytes. Patients who lack this enzyme are, therefore, immune deficient and vulnerable to frequent infections. Injections of PEG-ADA may increase the number of immune cells and reduce infections, but this enzyme replacement therapy is not a definitive cure. In addition, patients may become resistant or allergic to the drug. Gene therapy, in which a normal ADA gene is inserted into the patient's cells, attempts to correcting the underlying cause of disease.

Patients with SCID due to ADA deficiency may be eligible for this study. Patients may or may not have received enzyme replacement therapy or gene transfer therapy, or both. Participants will have follow-up visits at the National Institutes of Health in Bethesda, Maryland, at least once a year for a physical examination, blood tests, and possibly the following additional procedures to evaluate immune function:

1. Bone marrow sampling - A small amount of marrow from the hip bone is drawn (aspirated) through a needle. The procedure can be done under local anesthesia or light sedation.
2. Injection of small amounts of fluids into the arm to study if the patient's lymphocytes respond normally.
3. Administration of vaccination shots.
4. Collection of white blood cells through apheresis - Whole blood is collected through a needle placed in an arm vein. The blood circulates through a machine that separates it into its components. The white cells are then removed, and the red cells, platelets and plasma are returned to the body, either through the same needle used to draw the blood or through a second needle placed in the other arm.
5. Blood drawings to obtain and study the patient's lymphocytes.