OBJECTIVE: To investigate the prevalence of and twin pair concordance for endometriosis. DESIGN: A questionnaire survey incorporating validation. SETTING: An Australia-wide volunteer sample of female monozygotic (MZ) and dizygotic (DZ) twin pairs from the Australian National Health and Medical Research Council Twin Register. PATIENT(S): Twins were selected only on the basis of previous participation in twin research. INTERVENTION(S): Questionnaires were sent to 3,298 individuals. Information was requested from physicians named by consenting twins. MAIN OUTCOME MEASURE(S): Reported endometriosis, validated where possible by pathology or surgical report. RESULT(S): Three thousand ninety-six (94%) of the twins and 145 (82%) of the physicians responded to the survey. Two hundred fifteen twins reported endometriosis, for a prevalence rate of .07 among question respondents. Tetrachoric twin pair correlations for self-reported endometriosis (MZ: n = 854 and DZ: n = 493) were rMz = .46+/-.09 and rDz = .28 +/-.13. When available medical and pathology reports were included, they changed to rMz =.52 +/-.08 and rDZ = .19+/-.16, suggesting that 51% of the variance of the latent liability to endometriosis may be attributable to additive genetic influences. CONCLUSION(S): These findings support the hypothesis that genes influence liability to endometriosis.