Donor-cell leukemia post bone marrow transplantation is a rare event. Most of the cases reported to date have developed in cells from an HLA-matched sibling, who had no evidence of malignant disease before or following the occurrence of donor-origin leukemia. We describe a 17-year-old female who developed B-cell lymphoma 9 years following the occurrence of donor-origin acute myeloid leukemia in her brother for whom she had donated marrow. Cytogenetic analysis of the tumor revealed multiple chromosomal aberrations. The donor was heterozygous for the Ashkenazi mutation of Bloom's syndrome, suggesting that donor-type leukemia could have resulted from genomic instability in the donor cells.