We searched for trisomy 11 in acute myelogenous leukemia (AML) patients using the Japan Adult Leukemia Study Group (JALSG) AML-92 and -95 databases to clarify the clinical and hematologic features of a rare numerical chromosome abnormality. Among the sequentially registered patients of JALSG AML-92 (655 patients) and JALSG AML-95 (531 patients), chromosome findings were obtained for 1074 patients (90.6%); we found 5 patients with trisomy 11 as the sole abnormality. The patients were 4 women and 1 man with trisomy 11 AML, all aged more than 45 years (median, 52 years), with 4 M1 morphologies and 1 M2. No patients manifested hepatosplenomegaly or lymph node enlargement, and no central nervous system leukemia or extramedullary lesions were detectable. All showed positivity for CD13 (5/5), CD33 (5/5), CD34 (3/3), CD38 (2/2), and HLA-DR (5/5). Except for 1 patient, all achieved complete remission after 1 course of induction chemotherapy, but 2 relapsed after discontinuation of chemotherapy. A third case of relapse occurred during intensification of chemotherapy, and the patient underwent allogenic bone marrow transplantation but died from interstitial pneumonia.