Background: Chronic endometritis is a slight inflammation of the endometrium that is histologically diagnosed mainly by the presence of plasma cells in the endometrial stroma. In many previous clinical studies, the clinical outcomes were compared between the group cured with antibiotics and the persistent group, and the subjects were patients with recurrent implantation failure. However, antibiotics cannot be administered without establishing diagnostic criteria in advance. It is also difficult to purely evaluate the effect of chronic endometritis on implantation when the control group is defined as patients with recurrent implantation failure without chronic endometritis, since the pregnancy rate in patients with recurrent implantation failure will be lower due to the presence of causes other than chronic endometritis for implantation failure. For these reasons, there appear to be no uniform criteria based on clinical outcomes that are accepted worldwide.Methods:A prospective observational study was conducted in a single university from June 2014 to September 2017. Patients who underwent single frozen-thawed blastocyst transfer with a hormone replacement cycle after histological examination for the presence of chronic endometritis were enrolled. Participants with recurrent implantation failure, recurrent pregnancy loss, and diseases suspected to cause implantation failure were excluded. Four criteria were used to define chronic endometritis according to the number of plasma cells in the same group of patients: 1 or more plasma cells, 2 or more, 3 or more, or 5 or more in 10 high-power fields. Pregnancy rates, live birth rates, and miscarriage rates of the Non-chronic endometritis and the chronic endometritis groups defined with each criterion were calculated.Results: The pregnancy rate and live birth rate of Non- chronic endometritis was highest and all P values for pregnancy rates, live birth rates, and miscarriage rates were smallest when the diagnostic criterion of chronic endometritis was defined as the presence of one or more plasma cells in 10 high-power fields. Conclusion: Chronic endometritis should be diagnosed as the presence of one or more plasma cells in 10 high-power fields. According to this diagnostic criterion, chronic endometritis adversely affected the pregnancy rate and the live birth rate.