C/EBPalpha plays an essential role as a transcription factor in myeloid cell differentiation. Here, we describe a Japanese family in which two individuals with acute myeloid leukemia (AML) and one healthy individual had an identical 4-base pair insertion in the N-terminal region of CEBPA (350_351insCTAC), resulting in the termination at codon 107 (I68fsX107). The father and a son at diagnosis of AML had different in-frame insertion mutations in the C-terminal region of C/EBPalpha. These C-terminal mutations disappeared upon remission in both patients. Interestingly, the father showed different in-frame insertion mutations in the C-terminal CEBPA at the time of diagnosis and relapse. These data strongly suggest that the N-terminal C/EBPalpha mutation predisposes to the occurrence of a C-terminal C/EBPalpha mutation as a secondary genetic hit, causing AML.