Early results of unrelated cord blood transplantation (UCBT) for severe aplastic anemia (SAA) were poor with a high rate of engraftment failure. This was attributed to the combination of lower graft cell dose and intact host immune system. We performed UCBT in nine children (median age 9 years) with refractory SAA using increasingly immunosuppressive preparative regimens. The time from diagnosis to UCBT was 3.4-20 months (median age 7.2 years), with all children having failed at least one course of immunosuppression. Donor/recipient HLA matching was six of six (n=1), five of six (n=2) and four of six (n=6). The median nucleated cell dose infused was 5.7 x 10(7) cells/kg (range 3.5-20 x 10(7) cells/kg). Six patients were engrafted after the first UCBT. Two of the three patients without hematopoietic reconstitution were engrafted after a second UCBT. All children receiving >or=120 mg/kg of CY in the preparative regimen were engrafted. The median time to myeloid engraftment was 25 (17-59 days) days. Acute GVHD developed in two, and chronic GVHD in five patients. Five patients developed EBV viremia post transplant (lymphoproliferative disorder in three patients). At a median follow-up of 34 months, seven patients are alive and transfusion-independent. UCBT is a feasible treatment strategy for children with refractory SAA lacking a well-matched adult donor.