We report the case of the development of two different stages of the same clonal disorder in two patients sharing the same bone marrow due to a previous bone marrow allotransplant. The transplanted patient developed severe aplasia with myeloid blasts, different from those of the previously cured leukemia. Chimerism evaluated by microsatellite analyses confirmed a full donor phenotype. At the same time, the donor of the bone marrow transplantation developed a refractory anemia with excess blasts. We speculate on the presence of an undetectable pre-existing pathological clone in the transplanted bone marrow, which have evolved in the two patients.