Endometrial cancer is the most commonly diagnosed type of gynecological cancer, and obesity is a major risk factor for this disease. Despite rising rates of obesity and endometrial cancer, it is still unclear how obesity alters the benign endometrium and the endometrial hormonal response to promote carcinogenesis. We used an endometrial organoid model to investigate how adiposity predisposes women to develop endometrial cancer. We hypothesized that increased adiposity would directly alter endometrial gene expression and diminish the protective progesterone response of the benign endometrium. Scaffold-free, three-dimensional endometrial organoids consisting of both epithelial and stromal cells from benign endometrial tissue were generated. Organoids were cocultured with different numbers of adipocyte spheroids to represent different levels of adiposity and were treated with a menstrual cycle level of hormones (E2 + P4 +T) over 14 days. Endometrial organoid gene expression was investigated using RNA-seq. Coculturing endometrial organoids with 30 adipocyte spheroids in the presence of E2, P4, and T caused significant differential expression of pathways involved in metabolism, angiogenesis, chromatin modification, and metal ion response, compared to endometrial organoids cultured without adipocyte spheroids. We investigated the metal ion response pathways that were altered and found that many of the most significantly differentially regulated genes in the metal ion response pathways belonged to the metallothionein (MT) family of genes. The expression of MT1A, MT1M, MT1X, and MT2A decreased approximately 50% in endometrial organoids cocultured with 30 adipocyte spheroids. Furthermore, although the results were not statistically significant, we observed a downregulation of the progesterone-responsive genes HSD17B2, IGFBP1, PAEP, PRL, FOXO1, and SPP1 in the endometrial organoids cultured with 30 adipocyte spheroids. Our results show that culturing endometrial organoids in conditions of increased adiposity alters endometrial gene expression and may change the way the endometrium responds to progesterone. Metallothioneins, which are downregulated in the presence of adipocyte spheroids, are important modulators of zinc and copper ions and reactive oxygen species in the cell. Progesterone plays an important protective role against endometrial cancer. In summary, our data show that adipocytes may influence the progesterone response of the benign endometrium. Further investigation into the role of progesterone-driven metallothioneins in the endometrium is currently underway. Citation Format: Alina R. Murphy, J. Julie Kim. Understanding the influence of adiposity on the endometrium using human primary endometrial organoids [abstract]. In: Proceedings of the AACR Virtual Special Conference: Endometrial Cancer: New Biology Driving Research and Treatment; 2020 Nov 9-10. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(3_Suppl):Abstract nr PO030.