The cytogenetic, hematologic, and clinical characteristics of a 13-year-old girl with acquired t(6;9)(p23;q34) and dysmyelopoietic syndrome developing into acute myelomonocytic leukemia are described, bringing the total number of patients with t(6;9) and hematologic disease described in the literature up to 19. The diagnosis has been acute myeloid leukemia in the great majority of these patients; only four have had acute myelomonocytic leukemia. High resolution analysis at the 550 band stage localized the breakpoints in chromosomes #6 and #9 to p23 and 9q34.3, respectively. Previous investigations employing high resolution cytogenetics have mapped the typical 9q breakage site in chronic myeloid leukemia to 9q34.1. In situ hybridization studies have demonstrated that the cellular oncogene c-abl remains on the derivative 9q+ chromosome in t(6;9), whereas it is moved to the Ph marker in t(9;22). Thus, the combined data indicate that c-abl is located between 9q34.1 and 9q34.3, i.e., in subband 9q34.2 or its immediate vicinity.