BACKGROUND: The exact impact of full-staging lymphadenectomy on patients with primary mucinous epithelial ovarian carcinoma confined to the ovary is still unclear. In this study, we investigated the prognostic impact of lymphadenectomy covering both pelvic and para-aortic lymph nodes in patients with clinically-apparent stage I mucinous epithelial ovarian carcinoma, using data from multi-institutions under a central pathological review system and analyses with a propensity score-based method. METHODS: We conducted a regional multi-institutional retrospective study between 1986 and 2017. Among 4730 patients with malignant ovarian tumors, a total of 186 women with mucinous epithelial ovarian carcinoma were eligible. We evaluated differences in survival outcomes between patients with both pelvic and para-aortic lymphadenectomy and those with only pelvic lymphadenectomy and/or clinical lymph node evaluation. To analyze the therapeutic effects, the baseline imbalance between patients with both pelvic and para-aortic lymphadenectomy and others was adjusted with an inverse probability of treatment weighting using propensity score involving independent clinical variables. RESULTS: Fifty-five patients received both pelvic and para-aortic lymphadenectomy. With PS-based adjustment, both pelvic and para-aortic lymphadenectomy did not have additive effects regarding overall survival (P = 0.696) and recurrence-free survival (P = 0.978). Multivariate analysis similarly showed no significant impact of both pelvic and para-aortic lymphadenectomy on their prognosis. CONCLUSIONS: The effect of pelvic and para-aortic lymphadenectomy is limited for clinically-apparent stage I primary mucinous epithelial ovarian carcinoma as long as full peritoneal and clinical lymph node evaluations are conducted. The results of this study should be used as the basis for additional studies, including prospective trials.