A 63-year-old woman presented with a 2-month history of progressive dyspnea, weight loss, and a macular-papular rash over her back (panel A). Her medical history included breast cancer (treated 5 years ago with surgery and adjuvant chemotherapy) and chronic mild thrombocytopenia (baseline platelet count of 100 × 10 9/L). Her full blood count was normal, except for a platelet count of 76 × 10 9/L. Blood film examination appeared reactive. Chest X-ray and computed tomography (CT) scans of the chest/abdomen/pelvis were normal. The patient was discharged on antibiotics for a chest infection but was readmitted 1 month later with progressive dyspnea. A high-resolution CT chest scan now showed signs suspicious of lymphangitic spread of breast cancer. A skin biopsy was taken of her rash. It revealed blast-like cells (panels B and C; magnification ×10 and ×60, respectively; hematoxylin and eosin stain), which were negative for cytokeratins, CD34, CD56, CD117, and terminal deoxynucleotidyl transferase, but were positive for CD43, CD31, Bcl2, CD4, CD33, and myeloperoxidase (panel D; magnification ×20; myeloperoxidase stain), suggestive of cutaneous involvement of acute myelomonocytic leukemia.The diagnosis of secondary (therapy-related) acute myeloid leukemia (AML) was confirmed on bone marrow biopsy. Cytogenetics showed an abnormal hypotetraploid karyotype: 88<4n>,XXXX,−1,−2,+8,−9,−10,−11,−14,−15,−17,+mar1x3[13]/46,XX[2]. The patient deteriorated and died 6 days later. Hypotetraploidy (or near tetraploidy) has not previously been reported in secondary AML, but it is a rare finding in primary AML and is associated with a poor prognosis.