The transcription factor CCAAT/enhancer binding protein alpha (C/EBPa) encoded by the CEBPA gene, is crucial for the differentiation of immature granulocytes. Diminished or abnormal C/EBPa activity resulting from CEBPA gene mutations is widely known to contribute to the transformation of myeloid progenitors via reduction of their differentiation potential. The CEBPA mutations have been detected in approximately 7% of total acute myeloid leukemia (AML) and in 15% of those with intermediate-risk cytogenetics or those with normal karyotype. However, the age distribution of the patients with the CEBPA mutations and the immunophenotype of their leukemic cells are not known. Sequential studies of the CEBPA gene in AML patients are also limited. In this study, 104 patients with de novo acute myeloid leukemia (AML) were evaluated for the CEBPA mutation by direct sequencing. Excluding the silent mutations, 16 (15%) of the total 104 AML patients, 15 (25%) of the 61 patients with intermediate-risk cytogenetics and 11 (35%) of the 31 patients with normal karyotype showed CEBPA mutations, frequencies higher than those reported in the West. Further cloning and subsequent nucleotide sequence analysis revealed that 14 patients had heterozygous biallelic mutations: 11 had mutations involving both the N-terminal transactivation domain (TAD) and the C-terminal basic leucine zipper domain (bZIP) and three, in either the TAD region or the bZIP region. The remaining two patients had only one allele mutation in the TAD1 region. Most mutations in TAD region were repeat-number changes of simple sequence repeats and those in bZIP region were internal tandem duplications. Sequence analysis revealed that in the region spanning the bZIP mutations, there was hot spot for concensus topoisomerase II sites, which has also been shown in other AML-related mutations FLT3-ITD and MLL duplication. All but one patient with CEBPA mutations had M1 or M2 subtype of AML. The patients with CEBPA mutations had significantly higher incidences of CD7 (73%), CD15 (100%), CD34 (93%) and HLA-DR (93%) expression than others and the majority of them showed a distinct immunophenotype of the leukemic cells: HLA-DR+ CD7+ CD13+ CD14− CD15+ CD33+ CD34+. The incidence of the CEBPA mutation in children with AML was similar to that in adults. The CEBPA mutation was serially analyzed in 27 patients; the mutations disappeared at CR, but reappeared at relapse. No one developed novel mutation during the follow-up period. In conclusion, the CEBPA mutation may play an important role in the development, but not progression, of AML. The patients with the CEBPA mutations showed a distinct immunophenotype of the leukemic cells. Potential topoisomerase II cleavage sites locating in the bZIP region were first reported and we propose that this is relevant to the process of illegitimate recombination generating the internal tandem duplication pattern of bZIP mutations.