Allogeneic transplantation from an HLA-matched family member has been shown to be effective in reconstituting normal haemopoiesis in young people with severe cytopenias, classified as myelodysplastic syndrome (MDS) or severe aplastic anaemia (SAA). Unrelated donor transplant is a therapeutic choice for patients without a suitable family member donor. We report the outcome of seven patients < 20 years old with SAA and 10 with MDS treated with BMT from an HLA A,B DRB1 matched (n = 8) or A or B locus mismatched (n = 9) unrelated donor at the University of Minnesota between March 1988 and August 1995. Primary graft failure occurred in two patients and secondary graft failure in one, who was subsequently successfully engrafted with a second donor marrow infusion. Grades II-IV GVHD occurred in 10/16 (63%), and grades III-IV in 6/16 (37%) evaluable patients. Nine of the 17 patients (six with MDS and three with SAA) survive with full donor chimaerism, a median of 1.2 years post-BMT (range 3 months to 7 years). We recommend early referral for consideration of unrelated donor BMT for young patients with MDS, and patients with SAA without response to immunosuppression.