To estimate the relative contribution of genetic influences and prevalence on endometriosis.Analysis of self-reported data from a nationwide population-based twin registry.Not applicable.A total of 28,370 women, female monozygotic (MZ) or dizygotic (DZ) twins, who participated in either of two surveys (1998-2002 or 2005-2006).None.Self-reported endometriosis, validated by medical records.A history of endometriosis was reported by 1,228 female twins. The probandwise concordance was 0.21 for MZ and 0.10 for DZ twins. Higher within-pair (tetrachoric) correlation was observed among MZ (0.47) compared with DZ (0.20) twins. The best-fitting model revealed a contribution of 47% by additive genetic factors and the remaining 53% attributed to unique environmental effects.Our findings suggest both genetic and unique (nonshared) environmental influences on the complex etiology of endometriosis and support the hypothesis that genes have a strong influence on phenotypic manifestations of endometriosis.