We report a rare case of transient abnormal myelopoiesis (TAM) in a phenotypically normal neonate. The presence of a palpable hepatomegaly prompted in-depth laboratory tests, which revealed the presence of severe hyperleukocytosis, with blast cells present in a peripheral blood smear. Although no signs of Down syndrome were present, we suspected TAM. Further analysis identified a mutation in GATA1 along with the unique finding of two different trisomic cell lines, detected upon karyotyping; one with trisomy 21 only, and one with trisomies 21 and 22, which was present in a subpopulation of peripheral blood cells. These genetic abnormalities disappeared by the age of 6 months. The presence of two different trisomic clones may be an evidence of the polyclonal nature of TAM in this patient.