The most reliable chronic endometritis diagnosis is based on immunohistochemistry plasma cell identification in endometrial samples. Our study aimed to compare multiple myeloma oncogene 1 (MUM1) and syndecan-1/CD138 immunohistochemistry staining for chronic endometritis diagnosis among patients with recurrent pregnancy loss. We evaluated the presence of endometrial stromal changes. Fifty-four patients with a history of at least two intrauterine pregnancy losses underwent diagnostic hysteroscopy in the follicular phase of the cycle with endometrial aspiration biopsy. In all 54 cases, three successive sections were cut from each paraffin-embedded tissue block for hematoxylin and eosin (H&E), CD138 and MUM1 staining. The goal was to evaluate the level of agreement between the MUM1 and CD138 results and plasma cell detection rate in assessing the endometrial stromal changes. The concordance analysis between CD138 and MUM1 immunohistochemistry staining showed consistent results in 43 of 54 (79.6%) cases. The level of agreement was moderate, based on a Kappa value of 0.60. MUM1 immunostaining was positive for CE in more cases than CD138 staining, and this difference was statistically significant, showing a higher sensitivity of MUM1 in plasma cell detection (p=0.01). Endometrial stromal changes were observed in the majority of cases - 49/54 (90%). Samples without stromal changes were consistently negative for plasma cells using both CD138 and MUM1 staining. We demonstrated that MUM1 staining, used in conjunction with assessing endometrial stromal changes, contributes to a more accurate and comprehensive diagnosis of chronic endometritis.