Aim: To explore the relationship between NPAR and endometriosis in women over 20 years old Material and methods: We screened the data from National Health and Nutrition Examination Survey (NHANES) 1999–2006. A total of 4595 eligible women were included in this study. To estimate the independent association of NPAR with the risk of endometriosis, we used weighted multivariate logistic regression models and cubic spline analyses. Results: Among women over the age of 20 in the NHANES 1999–2006 database, NPAR values were significantly and positively associated with endometriosis. And patients in the highest quartile of NPAR values (Q4) had a prevalence of endometriosis that was approximately 2.19 times that of patients in the lowest quartile (Q1) (95% CI: 1.52-3.14, P <0.001). After multivariable adjustment, higher NPAR values were found to be associated with a higher prevalence of endometriosis (OR: 2.24, 95% CI: 1.55–3.23, P for trend = 0.001). Spline regression showed a non-linear correlation (P-non-linear ? 0.001) between NPAR and the incidence of endometriosis, which was N-shaped with a node value of 11.162 (P-overall ? 0.001). Conclusion: Our findings suggest that there is an association between NPAR and endometriosis, and lower NPAR values may be associated with a lower risk of endometriosis, which provides new ideas for the serological diagnosis of endometriosis