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Background: To examine the association between malignant peritoneal cytology and survival in women with early-stage endometrioid endometrial cancer., Methods: This is a retrospective cohort study using the Surveillance, Epidemiology, and End Results Program from 2010 to 2016. Women with stage I endometrioid endometrial cancer who had peritoneal cytology testing at hysterectomy were examined (N = 24,800). Characteristics and survival related to malignant peritoneal cytology were assessed. The propensity score inverse probability of treatment weighting was used to balance the measured covariates., Findings: Malignant peritoneal cytology was reported in 1081 (4.4%) women. In multivariable analysis, stage IB disease and moderately/poorly differentiated tumours were associated with an increased likelihood of malignant peritoneal cytology (both P < 0.05). In a weighted model, malignant peritoneal cytology was associated with decreased cause-specific survival (5-year rates, 92.1% versus 96.8%, hazard ratio [HR] 1.98, 95% confidence interval [CI] 1.56-2.52) and overall survival (89.4% versus 93.1%, HR 1.41, 95% CI 1.16-1.72). In sensitivity analyses, malignant peritoneal cytology was associated with decreased overall survival in the high-intermediate-risk group (5-year rates, 77.8% versus 83.6%, HR 1.57, 95% CI 1.20-2.06) and decreased cause-specific survival in the low-risk group (95.4% versus 98.0%, HR 1.64, 95% CI 1.01-2.68). In the high-intermediate-risk group with malignant peritoneal cytology, postoperative chemotherapy was associated with improved overall survival compared to whole pelvic radiotherapy (5-year rates, 82.7% versus 64.6%, HR 0.36, 95% CI 0.14-0.96). This association was not observed in negative cytology cases (81.5% versus 79.7%, HR 0.78, 95% CI 0.53-1.14)., Interpretation: Malignant peritoneal cytology may be associated with decreased survival in stage I endometrioid endometrial cancer., (Copyright © 2020 Elsevier Ltd. All rights reserved.)