Deletion of the short arm of chromosome 12 as the sole karyotypic abnormality was found in three cases of acute myeloblastic leukemia (AML). In contrast to what has previously been reported in the literature, none of our cases had an antecedent history of myelodysplasia or toxic exposure, and basophilia was absent. Two cases were terminal deoxynucleotidyl transferase (TdT) positive, suggesting involvement of an early progenitor cell.