To augment graft cell dose, we evaluated the safety of the combined transplantation of two partially HLA-matched umbilical cord blood (UCB) units. Five patients with transfusion-dependent thalassemia, median age 11.1 years (range 10-13.1), received 2 UCB units after myeloablative conditioning. Cord blood units were a 4/6-HLA-match or better with the recipient, and contained a minimum combined pre-freeze CD34 cell dose of 3.0 x 10(5)/kg. All patients engrafted at a median of 15 days (range 12-19). Four patients with durable trilineage engraftment showed acute grade I-III GVHD; none developed extensive chronic GVHD until the date of last contact. The median times to red blood cell transfusion independence and platelet engraftment were 32 and 49 days after transplant, respectively. With a median follow-up of 18.5 months (range 11-32), four patients transplanted with UCB from two different partially HLA-matched donors were transfusion-independent. Therefore, transfusion of two partially HLA-matched UCB units is safe, and may overcome the cell-dose barrier that limits the use of UCB in long-term recipients of multiple transfusions for thalassemia.