Oncogenic mutations in the KRAS2, NRAS, or FLT3 gene are detected in more than 50% of patients with de novo acute myeloid leukemia (AML). RAS mutations are also prevalent in de novo myelodysplastic syndrome (MDS), especially chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia. However, few studies have examined these genetic lesions in therapy‐related myeloid malignancies. Monosomy 7/del(7q) and monosomy 5/del(5q) represent the most common cytogenetic abnormalities in therapy‐related MDS and AML (t‐MDS/t‐AML) and are strongly associated with prior exposure to alkylating agents. Mutational analysis of bone marrow specimens from a well‐characterized cohort of 26 t‐MDS/t‐AML patients with abnormalities of chromosomes 5 and/or 7 revealed 3 with RAS mutations. Further analyses of 23 of these cases uncovered one FLT3 internal tandem duplication and five TP53 mutations. The four patients with RAS or FLT3 mutations had monosomy 7, including one with abnormalities of chromosomes 5 and 7. One specimen demonstrated mutations in both KRAS2 and TP53. RAS and FLT3 mutations, which are thought to stimulate the proliferation of leukemia cells, appear to be less common in t‐MDS/t‐AML than in de novo AML, whereas TP53 mutations are more frequent. © 2004 Wiley‐Liss, Inc.