Reduced-intensity conditioning may reduce transplantation-related mortality in high-risk adults undergoing hematopoietic transplantation. We investigated unrelated donor umbilical cord blood (UCB) transplantation after such conditioning in 43 patients (median age, 49.5 years; range, 22-65 years) with a primary end point of donor engraftment. The first 21 patients received busulfan 8 mg/kg, fludarabine 200 mg/m², and 200 cGy of total body irradiation (Bu/Flu/TBI). Subsequent patients (n = 22) received cyclophosphamide 50 mg/kg, fludarabine 200 mg/m², and 200 cGy TBI (Cy/Flu/TBI). UCB grafts (93%) were 1-2 HLA antigen–mismatched with the recipient and contained a median cryopreserved cell dose of 3.7 × 10⁷ (range, 1.6 × 10⁷-6.0 × 10⁷) nucleated cells per kilogram of recipient body weight (NC/kg). Graft versus host disease (GVHD) prophylaxis was cyclosporin A to day 180 plus mycophenolate mofetil to day 30. The cumulative incidence of sustained donor engraftment was 76% (95% confidence interval [CI], 56%-96%) for Bu/Flu/TBI recipients and 94% (95% CI, 84%-100%) for Cy/Flu/TBI recipients. The median day of neutrophil recovery (at least 0.5 × 10⁹/L) for engrafting Bu/Flu/TBI recipients was 26 days (range, 12-30 days) and for Cy/Flu/TBI recipients was 9.5 days (range, 5-28 days). Incidence of grades III-IV acute GVHD was 9% (95% CI, 1%-17%), and survival at 1 year was 39% (95% CI, 23%-56%). These data demonstrate that 0-2 antigen mismatched UCB is sufficient to engraft most adults after reduced-intensity conditioning and is associated with a low incidence of severe acute GVHD.