We review clinical outcome and immune reconstitution in a consecutive series of 74 infants with severe T cell immunodeficiency who received hematopoietic cell transplantation (HCT) from January 1991 to May 2003. Fifty-three patients (71.6%) are alive. Results were significantly better for recipients of HCT from HLA-matched related donors (100% survival) and unrelated donors (86.4%) than from mismatched related donors (51.6%). A detailed analysis of immune reconstitution and clinical status was performed in 49 surviving patients, most of which have attained robust T and B cell reconstitution and are in very good clinical conditions. No cases of late deaths or of chronic graft-versus-host disease (GvHD) have been observed. However, infections and autoimmunity at >1 year after HCT have been observed in a significant number of patients. Persistence of a low number of circulating naive T cells and long-term requirement for intravenous immunoglobulin were associated with a higher incidence of clinical events.