<h3>Study Objective</h3> The objective of our study is to further investigate the role of endometriosis on subsequent diagnosis of endometrial hyperplasia and malignancy (EHM). <h3>Design</h3> Retrospective cohort study <h3>Setting</h3> Single academic institution <h3>Patients or Participants</h3> All cases of histologically diagnosed endometriosis were evaluated between 2005 and 2020. Women over 18 years of age were included. All endometrial malignancies were included for analysis. Patients with a history of hysterectomy were excluded. <h3>Interventions</h3> No interventions were performed <h3>Measurements and Main Results</h3> Demographic information and past medical and surgical history were assessed. For the EHM cohort, oncologic data, endometriosis stage, patient age and body mass index (BMI) at time of diagnosis were collected. A total of 2,122 women with endometriosis were included. Of these, 198 (9.3%) were found to have co-existing eutopic endometrial hyperplasia (n=56) or malignancy (n=142). At the time of oncologic diagnosis, the mean age was 55.6 years and mean BMI was 33.1. Within the EHM group, 82.3% of endometriosis diagnoses were made at the time of their surgical management and the majority were stage 1 by ASRM criteria (70.0%). When comparing characteristics between those with and without a diagnosis of EHM, there was no significant difference in race, ethnicity, tobacco use, prior diagnosis of PCOS or diagnosis of breast or colon cancer. Significant differences were seen in type 2 diabetes (p<0.0001), hypertension (p<0.0001), prior use of tamoxifen (p=0.0021), gravidity (p=0.0331) and parity (p<0.0001). Statistical analysis was performed using chi-square and Mann-Whitney tests. A p-value of <0.05 was considered significant. <h3>Conclusion</h3> Compared to the population prevalence of less than 1% of both endometrial hyperplasia and malignancy, our study suggests that endometriosis may be a predictor of subsequent diagnosis of endometrial cancer and malignancy. Further research is needed to control for known confounders of endometrial hyperplasia and malignancy.