Dedifferentiated endometrial carcinoma, which is defined microscopically as the co-existence of undifferentiated carcinoma and grade 1 or 2 endometrioid adenocarcinoma, is an aggressive type of cancer regardless of the percentage of undifferentiated components. It is reported that undifferentiated carcinoma comprises 9% of endometrial carcinoma. The percentage of dedifferentiated endometrial carcinoma has been hypothesized to be 40% of undifferentiated carcinoma. A precise pathological diagnosis is essential for defining the appropriate therapeutic approach and prognosis. Furthermore, since there is an association between dedifferentiated endometrial carcinoma and Lynch syndrome, it is important to identify the patient's genetic background. The current case report presents three cases of dedifferentiated endometrial carcinoma treated in our hospital. In immunohistochemical staining for DNA mismatch-repair (MMR) proteins in dedifferentiated endometrial carcinoma, the components of undifferentiated carcinoma demonstrated a loss of MMR protein expression, and it is suspected that there may be a germline mutation in these cases. Therefore, Lynch syndrome should be suspected and the appropriate genetic approaches in cases of dedifferentiated endometrial carcinoma should be considered.