We herein report a rare chromosomal abnormality observed in an acute promyelocytic leukemia (APL) patient. She had several APL derivative clones including a clone with i(17)(q10) abnormality, which consists of two kinds of structural abnormalities, a cryptic translocation of t(15;17) and an isochromosome of 17q. Although an obvious microscopic t(15;17) change was not observed on either arms of the isochromosome, PML/RARα fusion signals were detected on an interphase fluorescence in situ hybridization analysis. By several cytogenetic analyses of her bone marrow cells, it was confirmed that the i(17)(q10) clone was derived from the classic t(15;17) clone via another intervening clone, cryptic t(15;17).