<p>This study aimed to investigate whether chronic endometritis (CE) affects pregnancy outcomes in non-assisted reproductive technology (non-ART) treatments, including timed intercourse (TI) and intrauterine insemination (IUI). This prospective observational study included 50 patients who underwent conventional infertility treatment at Yamagata University Hospital between April 2020 and March 2022. Endometrial tissue samples were collected during hysteroscopic screening and assessed for CE using CD138 immunostaining. CE was diagnosed when ≥ 5 CD138-positive plasma cells were identified in at least one of 10 randomly selected high-power fields. Pregnancy outcomes were compared between CE-positive and CE-negative groups, and generalized estimating equation analyses were performed to account for repeated treatment cycles within the same patient. Among the 50 participants, 27 were CE-positive and 23 were CE-negative, yielding a CE prevalence of 54.0%. A total of 146 treatment cycles were analyzed. Clinical pregnancy rates were 10.6% in the CE-negative group and 13.8% in the CE-positive group, and live birth or second-trimester termination rates were 10.6% and 12.5%, respectively. In crude and adjusted analyses, CE was not significantly associated with clinical pregnancy or live birth or second-trimester termination. CD138-positive plasma cell counts were not clearly associated with pregnancy outcomes. In this limited single-center cohort, CE was not significantly associated with poorer pregnancy outcomes in TI or IUI treatments. However, because of the limited sample size and small number of pregnancy events, these findings should be interpreted cautiously. Larger studies are needed to determine whether CE screening or treatment has clinical value during non-ART fertility care.</p>