Lymphoblasts of acute lymphoblastic leukemia origin reappeared in a male patient 34 years after the initial diagnosis. Comparison of DNA profiles of the initial and reappeared lymphoblasts revealed a partially identical sequence, indicating the possibility that these lymphoblasts may have been present as preleukemic stem cells at the time of diagnosis and remained dormant for a long period until additional events conferred proliferative activity to these dormant preleukemic stem cells. Thus, in some cases of very late relapse, the reappearance of lymphoblasts may not be due to the relapse of the original leukemic clone, but to a clonal progression of a pre-existing subclone derived from preleukemic stem cells.