Abstract： Background::Postoperative chylous ascites is an infrequent condition after colorectal surgery and is easily treatable. However, its effect on the long-term oncological prognosis is not well established. This study aimed to investigate the short-term and long-term impact of chylous ascites treated with neoadjuvant therapy followed by rectal cancer surgery and to evaluate the incidence of chylous ascites after different surgical approaches.Methods::A total of 898 locally advanced rectal cancer patients treated with neoadjuvant chemoradiotherapy followed by surgery between January 2010 and December 2018 were included. The clinicopathological data and outcomes of the patients with chylous ascites were compared with those of the patients without chylous ascites. The primary endpoint was recurrence-free survival (RFS). To balance baseline confounders between groups, propensity score matching (PSM) was performed for each patient with a logistic regression model.Results::Chylous ascites was detected in 3.8% (34/898) of the patients. The incidence of chylous ascites was highest after robotic surgery (6.9%, 6/86), followed by laparoscopic surgery (4.2%, 26/618) and open surgery (1.0%, 2/192, P = 0.021). The patients with chylous ascites had a significantly higher number of lymph nodes harvested (15.6 vs. 12.8, P = 0.009) and a 3-day longer postoperative hospital stay ( P = 0.017). The 5-year RFS rate was 64.5% in the chylous ascites group, which was significantly lower than the rate in the no chylous ascites group (79.9%; P = 0.007). The results remained unchanged after PSM was performed. The chylous ascites group showed a nonsignificant trend towards a higher peritoneal metastasis risk (5.9% vs. 1.6%, P = 0.120). Univariate analysis and multivariate analysis confirmed chylous ascites (hazard ratio= 3.038, P < 0.001) as an independent negative prognostic factor for RFS. Conclusions::Considering the higher incidence of chylous ascites after laparoscopic and robotic surgery and its adverse prognosis, we recommend sufficient coagulation of the lymphatic tissue near the vessel origins, especially during minimally invasive surgery.