Trisomy 8 is a frequently acquired cytogenetic abnormality in myeloid malignancies, but may also represent a constitutional chromosome abnormality with a wide phenotypic variation. We report a case of myelodysplastic syndrome (MDS) that developed in a child with trisomy 8 mosaicism and normal phenotype. Bone marrow (BM) cells all showed trisomy 8 with additional clonal abnormalities in most cells. Based on the present case and a review of previously published cases of myeloid malignancies in patients with trisomy 8 mosaicism, it appears likely that the malignant cells developed from the trisomic cell population, suggesting that constitutional trisomy 8 may be a predisposing condition to myeloid malignancies. Trisomy 8 in malignant cells is usually considered an acquired abnormality, but this implies a risk of ignoring a constitutional trisomy 8 mosaicism. Examination for constitutional trisomy 8, despite a normal phenotype, may therefore be warranted in hematologic malignancies with trisomy 8 of BM cells to evaluate further the possible association and to preclude erroneous use of trisomy 8 as a tumor marker.