Trisomy 4 is a recently defined chromosomal aberration in acute leukemia. The first reports suggested that this cytogenetic anomaly belongs to the M4 leukemia subtype of the FAB classification, but recent reports have described this alteration in a wider spectrum of leukemia subtypes. Here we report two cases of trisomy 4 as the sole chromosome anomaly: one was observed in a patient with acute biphenotypic leukemia with B-lineage markers and the second in a patient diagnosed with acute minimally differentiated myeloid leukemia (M0) with myelodysplastic features. To our knowledge these are, respectively, the first and second reports of trisomy 4 as the sole chromosomal anomaly in these leukemia subtypes.