We evaluated the incidence of trisomy 11 in acute myeloblastic leukemia (AML) and its correlation with the most relevant clinical, biological, and immunophenotypic disease characteristics in a total of 399 consecutive AML patients. Trisomy 11 was found in 15 patients (3.8%), in 3 of them as the sole abnormality. Median age was 68 years (range 48-87); 87% of patients were older than 60 years. Seven patients displayed multilineage dysplasia. Cytogenetics showed an association with trisomy 8 in six patients, and in five cases with a del(5q); nine patients had complex karyotypes. In all cases, in situ hybridization studies revealed three copies of the MLL gene, but no rearrangements or tandem duplications of MLL. Immunophenotypic analysis of blast cells showed a constant immature immunophenotypic profile with CD34 or CD117 expression (or both) in all cases analyzed. The complete remission rate was 43%; median survival time was only 2 months.