Some cases of myeloproliferative disorders (MPD) could be transformed into acute myeloid leukemia (AML), but the cell differentiation process of MPD into AML has not yet been observed in vivo. This study was aimed to reveal this differentiation process. The flow cytometry was used to analyse the immunophenotype of differentiated cells of 2 MPD cases who developed into AML in a short time. The reports showed that the different MPD-AML subclones are presented when the MPD cells that proliferate slowly in vivo become the AML blast cells that proliferate rapidly. It is concluded that understanding the process of MPD crisis will help the MPD-AML early diagnosis and treatment.