We report the outcome of 12 children who underwent unrelated cord blood transplant (U-CBT) in a single institution between February 1997 and July 1998. The 1 year event-free survival was 67% (95% CI of 26%). Four children died with infectious complication as cause of death in three cases. Immune reconstitution was studied during first year post transplant by assaying total lymphocyte counts, B cells, NK cells and T cell subsets in the eight disease-free surviving patients. We observed a prompt recovery of CD19+ cell number which was greater than 500/microl at 9 months for all patients except the one with severe cGVHD. B cells constituted the predominant lymphocyte subset at 6 and 9 months post transplant with normal or elevated B cell numbers according to normal paediatric range. We noted normal serum immunoglobulin levels at 6 months post transplant for IgA and IgM and at 9 months for IgG. The CD3+ cell count and particularly the CD3+CD8+ T cell subset remained depressed until 12 months post transplant. Six months after unrelated CBT, seven out of eight patients had less than 100 CD3+CD8+ cells/microl. CD3+CD4+ cell recovery was less impaired with all children achieving an absolute count of CD3+CD4+ cells greater than 200/microl during the first year in a median of 5 months. The percentage of NK cells was elevated during the first 6 months after CBT but their absolute count remained within the normal range. Bone Marrow Transplantation (2000) 25, 53-57.