A 22-yr-old man with acute myelocytic leukemia received a bone marrow transplant from a genotypically HLA-identical female sibling after cyclophosphamide preparation. He remained in complete remission for 18 mo, when he developed a chloroma in the perineum. The chloroma was treated with local radiotherapy. The chloroma recurred 8 mo later and was treated with radiotherapy followed by combination chemotherapy. At 34 mo after transplant, marrow relapse and chloroma were documented. The first chloroma contained host cells by fluorescent Y-chromatin body analyses of interphase nuclei. All metaphase cells and karyotypes from peripheral blood and marrow samples showed no evidence of host cells from 3 wk after transplant through the time of marrow relapse. Data from autosomal and sex chromosome studies indicate that the marrow relapse occurred in cells of donor origin. A new consistent chromosome abnormality [45, X, -X, t(8;21) (q22; q22)] was observed in a majority of donor cells. The patient received a second bone marrow transplant from the same donor after preparation with busulfan and cyclophosphamide and attained a complete remission with full hematologic engraftment.