Abstract Peritoneal lavage is a routine procedure in which normal saline solution is administered into the abdominal cavity and the washings are cytologically examined for cancer cells. Peritoneal lavage often strips protective mesothelial linings and can promote formation of peritoneal adhesions. However, the impact of peritoneal lavage on the omentum, which is also exposed to this procedure, is unclear. The omentum is a fatty tissue that suspends from the stomach and contains highly vascularized immune cell structures called milky spots. Because circulating ovarian and gastric cancer cells have strong predilection for implanting in milky spots, it is important to understand the impact of peritoneal lavage on the omentum, as not all patients with these cancers undergo omentectomy. In this study, we administered clinically relevant amounts of normal saline solution to immunecompetent mice to mimic peritoneal lavage, and evaluated the absorption of saline solution and the gross architecture and immune cell composition of the omentum and other peritoneal fat tissues. As compared to other peritoneal fat tissues, the omentum showed the highest capability to absorb saline solution. Following administration of saline, a dramatic expansion in milky spots and increased vessel density were observed in the omentum. Immunophenotyping and functional studies revealed that these cellular changes were due in part to the expansion of a sub-population of peritoneal macrophages. Furthermore, studies using mouse allograft models of ovarian cancer revealed that saline administration stimulated the implantation of circulating cancer cells on to the omentum. Collectively, these findings raise the possibility that peritoneal lavage ‘primes’ the omentum to be conducive for colonization by occult cancer cells that circulate in the peritoneal fluid. Citation Format: Hironari Akasaka, Honami Naora. Impact of peritoneal lavage on omentum immune cell dynamics and metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2521.