AbstractAn identical chromosome abnormality, which appears to be derived from a 1;7 translocation [ +der(1), t(1;7)(p11;p11)], was observed in the bone marrow of 12 patients with various hematologic disorders at the Mayo Clinic from 1980 to 1984. At the time of cytogenetic evaluation, nine of the patients had either a myeloproliferative disorder or a myelodysplastic syndrome, one had an acute leukemia, and two had treated multiple myeloma with no morphologic evidence of evolving myeloproliferative disease. Of the 11 patients for whom information about previous therapy was available, seven had a history of exposure to chemotherapy for a previous malignant disorder; we were unable to establish whether the remaining patient had had prior treatment. This study suggests a possible relationship between +der(1), t(1;7)(p11;p11) and some treatment‐induced hematologic disorders. Such chromosome abnormalities may be the result of a reciprocal chromatid translocation and adjacent I segregation of a quadriradial configuration in metaphas