SCN is characterized by neutropenia, life-threatening infections, and progression to myelodysplastic syndrome/acute myelogenous leukemia. The only curative option is SCT, but few reports using UCB as a stem cell source exist. Here, we report two SCN patients transplanted with UCB. Patient 1 was transplanted at seven yr of age due to increasingly large injections of G-CSF (>100 microg/kg/day) and the risk of developing leukemia. He engrafted promptly and is clinically well and immune reconstituted >2 yr post-transplant. Patient 2 underwent UCB SCT at nine months of age for recurrent severe infections, despite high doses of G-CSF. He rejected his first graft, having 100% host cells on day +35, and immediately underwent a second UCB SCT. He engrafted but experienced late graft rejection six months after the second transplant. He received a third UCB SCT following a more immunosuppressive conditioning regimen. His course was complicated by HHV-6 viremia and gut GVHD, but he is now clinically well and has 99% donor engraftment >20 months post-transplant. We conclude that UCB is an acceptable stem cell source for SCN patients, but conditioning must be adequately immunosuppressive to ensure engraftment in patients without prior chemotherapy.