Abstract Background: Unrelated umbilical cord blood has emerged as an alternative stem cell source for allogeneic transplantation for patients with haematological malignancies, but in adults is limited by the low number of stem cells present in banked cord blood units. We report our experience with double cord blood transplants for adult patients. The aim of the study was to compare the outcomes of double unrelated cord blood transplants in adults with poor prognosis haematological diseases with single cord blood transplants. Methods: Eleven patients, median age 27 years and median weight 69 kg, received transplants of two partially matched unrelated cord blood units after myeloablative conditioning therapy. Results: Neutrophil recovery to 0.5 × 109/L was seen by median day 32 (18–53), and platelet recovery to 50 × 109/L by day 91 (56–381). These results were not significantly different from those reported in patients receiving single cord blood transplants. Acute graft-versus-host disease of grades II–IV was seen in four patients, but no chronic graft-versus-host disease occurred. Transplant-related complications were responsible for the deaths of five patients in the first 3 months post-transplant, whereas two patients died of relapse of their haematological malignancy. Four patients survive, disease free, 17–33 months post-transplant. Conclusion: Transplantation using two partially matched unrelated cord blood units did not appear to result in improvements either in engraftment or survival, as compared with a previous cohort of patients receiving single cord blood units. Further strategies appear to be needed to reduce the duration of severe neutropenia and reduce the high transplant-related mortality in these patients.