No chromosomal rearrangements have been identified as specifically associated with minimally differentiated acute myeloid leukemia (AML-M0). Several research groups studied the cytogenetic features of AML-M0 and found that as much as 81% of patients with AML-M0 had chromosomal rearrangements; primarily -5/5q- and/or -7/7q- deletions or translocations involving 12p. A patient, who was diagnosed with AML-M0 eighteen months ago, was referred for cytogenetic evaluation for possible AML relapse. A subtle, cryptic t(5;9)(q35.3;q34.3), plus a deletion of the RB1 gene were detected in 18 out of 20 cells analyzed by FISH utilizing the TelVysion assay kit. To rule out the possibility that these chromosomal changes were related to the relapse of AML in this case, we repeated the same FISH test on the specimen at initial diagnosis before any treatment. The same abnormalities were found. To our knowledge, this is the first case reported with subtelomeric t(5;9)(q35.3;q34.3) and the deletion of the RB1 gene in a patient with AML-M0. Whether the t(5;9) combined with the deletion of the RB1 gene plays an important role in the development of AML-M0 warrants further investigation.