Seventy-nine acute myeloid leukemias (AML) and myelodysplastic syndromes without cytogenetic evidence of 12p aberrations were investigated by fluorescence in situ hybridization with probes for ETV6 and CDKN1B (previously called TEL and KIP1, respectively) to ascertain whether abnormalities of these genes are frequently undetected by standard chromosome banding analyses and, if so, whether they are associated with specific karyotypic patterns and morphologic features. One of sixty cytogenetically aberrant myeloid malignancies, an AML with a complex karyotype including del(5q) and del(20q), showed a hemizygous interstitial deletion of the ETV6 and CDKN1B loci. No concomitant rearrangement of the other ETV6 allele was detected. Two of nineteen cytogenetically normal AML displayed a hemizygous interstitial deletion involving CDKN1B, but not ETV6. Thus, cryptic deletions of these genes seem to be rare in cytogenetically abnormal myeloid malignancies without 12p aberrations (2%), whereas they may be more frequent in karyotypically normal AML (10%). Furthermore, the present findings show that the deletions may be narrow, not including the ETV6 gene, and indirectly suggest that CDKN1B, or a closely located genomic segment, is the target of 12p deletions.