Objective: To investigate the correlation of prepregnancy dietary inflammatory index (DII) with obstetric outcomes in women with endometriosis. Design: Prospective cohort study Setting: Japan Population: We identified 88,398 Japanese women (n=85,149 without endometriosis and n=3,249 with endometriosis) who were recruited in the Japan Environment and Children’s study (JECS) between January 2011 and March 2014. Methods: Participants were categorised according to DII quintiles (Q1 and Q5 were the most pro-inflammatory and most anti-inflammatory groups, respectively) and stratified according to the presence or absence of endometriosis. Women with endometriosis were further categorised based on conceptions after assisted reproductive technology. Main outcomes were preterm birth (PTB) and low birth weight (LBW) infant. A multiple logistic regression model was used to estimate the effect of anti-inflammatory diet on PTB before 37 or 34 weeks and LBW <2500 g or 1500 g. Results: In women with endometriosis without ART, Q5 significantly decreased the risk of PTB before <34 weeks (aOR 0.25, 95% CI 0.07–0.83) and LBW <1500 g (aOR 0.07, 95% CI 0.01–0.60). Conclusions: This study suggested a distinctive effect of anti-inflammatory diet on more severe obstetrics outcomes, specifically PTB before 34 weeks and LBW <1500 g, among women with endometriosis. Moreover, preconception lifestyle may improve perinatal mortality and morbidity among women with endometriosis. Funding: None