Haematopoietic stem cells in umbilical cord blood are an attractive target for gene therapy of inborn errors of metabolism. Three neonates with severe combined immunodeficiency were treated by retroviral–mediated transduction of the CD34⁺ cells from their umbilical cord blood with a normal human adenosine deaminase complementary DNA followed by autologous transplantation. The continued presence and expression of the introduced gene in leukocytes from bone marrow and peripheral blood for 18 months demonstrates that umbilical cord blood cells may be genetically modified with retroviral vectors and engrafted in neonates for gene therapy.