In this case report, we describe successful BMT with RIC in a patient with delayed-onset ADA deficiency. A three-yr-old Japanese boy was diagnosed with delayed-onset ADA deficiency because of recurrent bronchitis, bronchiectasia, and lymphopenia. In addition, autoimmune thyroiditis and neutropenia were present. At four yr of age, he underwent BMT with a RIC regimen, including busulfan and fludarabine, from an HLA-identical healthy sister. Engraftment after BMT was uneventful without GVHD. Decreased ADA levels in blood immediately increased following BMT, and the patient was disease-free 13 months after BMT. These results suggest that BMT with RIC may sufficiently restore immune regulation in delayed-onset ADA deficiency. A longer follow-up period is needed to confirm these observations.