We describe the successful unrelated cord blood transplantation in two patients affected by a Zap-70 deficiency and an Omenn-like syndrome, respectively. The patients were hospitalised for recurrent infections at the age of 13 and 2 months, respectively. An unrelated cord blood unit was found for each. The conditioning regimen was cyclophosphamide, busulfan and antithymocyte globulin. The total number of infused cells was 15.1 x 10(7)/kg and 17 x 10(7)/kg, respectively. Neutrophil engraftment was achieved on days +15 and +23, and platelet count >50 x 10(9)/l was achieved on days +21 and +52, respectively. One patient presented acute Graft-versus-host disease (GVHD) grade I and the other grade III. Chimerism was mixed and full donor. Normal lymphoproliferative response to mitogens and alloantigens was detectable at 6 months for both. No chronic GVHD was observed in either. The patients are alive and well at 53 and 15 months after transplantation. In conclusion, umbilical cord blood represents a valid alternative source of haemopoietic stem cells.