High treatment-related mortality (TRM) and high graft failure rate are serious concerns in HLA-mismatched umbilical cord blood (UCB) transplantation with myeloablative conditioning. We conducted a prospective trial of dual UCB transplantation using modified myeloablation consisting of total-body irradiation (TBI; 1350 cGy) and fludarabine (Flu) (160 mg/m²). Twenty-seven patients (median age, 33 years; range: 20-58 years) with hematologic malignancies were enrolled. The median combined cryopreserved total nucleated cell (TNC) dose was 4.3 × 10⁷/kg (range: 3.2-7.7 × 10⁷/kg). The cumulative incidences of neutrophil (≥500/μL) and platelet (≥50,000/μL) engraftment were 80% (95% confidence interval [CI], 58%-91%) and 68% (95% CI, 46%-83%), respectively. Among engrafted patients, a single cord blood unit was predominant by 100 days posttransplantation. A higher cryopreserved and infused TNC dose and infused CD3⁺ cell dose were significant factors associated with the predominant UCB unit (P = .032, .020, and .042, respectively). TRM and relapse rates at 2 years were 28% (95% CI, 12%-47%) and 20% (95% CI, 7%-37%), respectively. Cumulative incidences of grades II–IV and grades III–IV acute graft-versus host disease (aGVHD) were 37% (95% CI, 20%-55%) and 11% (95% CI, 3%-26%), respectively, and that of chronic GVHD (cGVHD) was 31% (95% CI, 15%-49%). With a median follow-up of 23 months, overall survival (OS) and disease-free survival (DFS) rates at 2 years were 58% (95% CI, 34%-75%) and 52% (95% CI, 29%-70%), respectively. This study supports the use of TBI 1350 cGy/Flu as an alternative to conventional myeloablative conditioning for dual UCB transplantation.