The present study aimed to determine the frequency of mismatch repair (MMR) protein expression loss, as identified using immunohistochemistry (IHC), in tumor cells of endometrial cancer patients and the potential associations between this loss of expression and various clinicopathological characteristics. The preparations were considered positive if tumor cells showed immunoreactivity that was equal to or stronger than that of positive controls and negative if tumor cells completely lost immunoreactivity. MMR proficiency was defined as positive IHC staining of all four proteins [MutL homolog 1 (MLH1), MutS homolog 2, MutS homolog 6 and PMS1 homolog 2 (PMS2)]. If at least one of them showed negative IHC staining, this was interpreted as mismatch repair protein deficiency (dMMR). A total of 154 patients who met the criteria were included in this study. dMMR was observed in 54 (35.1%) patients in the study group. The MLH1 and PMS2 proteins were the most frequently lost, observed in 44 (28.8%) and 43 (27.9%) patients, respectively. Patients with dMMR were significantly older. However, there were no observed associations between dMMR and other clinicopathological factors. In conclusion, a notable association between the expression of MMR proteins and the age of the patient was observed in this cohort. No significant associations were detected between other clinical, surgical or pathological factors and MMR protein expression.