AbstractBackground: Primary immunodeficiency diseases (PID) are rare but have a high associated risk of death from overwhelming infection in early childhood. Stem cell transplantation (SCT) can be curative for PID, but standardized protocols for each disease have not yet been established.Methods: Between May 1995 and May 2005, nine patients diagnosed with a PID received SCT at the Department of Pediatrics, Hokkaido University Hospital. The median age of the patients (eight boys and one girl) was 1.0 year (range: 6 months–4 years). Five patients had Wiskott–Aldrich syndrome (WAS), three had severe combined immunodeficiency (SCID), and one had X‐linked hyper‐IgM syndrome (X‐HIGM). Four patients received bone marrow transplantation (BMT), and five received cord blood stem cell transplantation (CBSCT). All patients, including those with SCID, received a conditioning regimen: six (WAS and X‐HIGM) received a myeloablative conditioning regimen, and three (SCID) received a reduced‐intensity conditioning regimen.Results: All the patients are alive and have stable, complete chimerism, based on a median follow‐up period of 4 years. Moreover, all patients have good immune reconstitution, and none required immunoglobulin replacement therapy. Two patients had significant acute graft‐versus‐host disease (GVHD), and three patients had chronic GVHD. Four of the nine patients developed cytomegalovirus (CMV) infection after SCT.Conclusion: The transplantation procedures appear to have provided a permanent cure in nine PID patients. Early diagnosis and prompt performance of SCT with an optimal donor and conditioning regimen contributed to the favorable outcomes.