Adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID) was the first inherited disease treated with gene therapy. The pilot gene therapy studies demonstrated the safety, therapeutic potential and limitations of ADA gene transfer into hematopoietic cells using retroviral vectors. This review describes the latest progress in ADA-SCID dinical trials using peripheral blood lymphocytes (PBLs) and hematopoietic stem cells (HSCs). PBL gene therapy was able to restore T-cell functions after discontinuation of ADA enzyme replacement therapy, but only partially corrected the purine metabolic defect. The development of improved HSC gene transfer protocols, combined with low intensity conditioning, allowed full correction of the immunological and metabolic ADA defects, with clinic benefit. These results have important implications for future applications of gene therapy in other disorders involving the hemapoietic system.