We analyzed 23 patients with myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML) showing der(1;7)(q10;p10) (hereafter der(1;7)), to identify the exact predictive factor of this cytogenetic change. Eight (34.8%) patients, including 6 MDS and 2 AML patients, had a prior history of genotoxic exposure, especially radiation and/or antimetabolites. Patients with der(1;7) consisted of three groups; one-third of patients had a prior history of genotoxic agents, one-third had additional cytogenetic changes at the time of MDS/AML diagnosis without prior exposure history, and the remaining one-third had neither prior exposure history nor additional cytogenetic changes. The current study demonstrated that the poor outcome of MDS/AML with der(1;7) is due to the high frequency of associated risk factors, i.e., prior history of genotoxic exposure, the presence of additional cytogenetic changes, or both. Identification of prognostic disadvantage might be required for appropriate strategy in managing MDS/AML patients with rare der(1;7) abnormality.