OBJECTIVE: The aim of this study was to investigate the association between early menstrual characteristics, before symptom onset, and later diagnosis of endometriosis. STUDY DESIGN: This was a case-control study of 268 Australian women with surgically confirmed moderate-to-severe endometriosis (cases) and 244 women without endometriosis (controls). Early menstrual cycle characteristics, before age at symptom onset, were analyzed. RESULTS: Menarche after age 14 years was strongly and inversely associated with endometriosis (odds ratio, 0.3; 95% confidence interval, 0.1-0.6). A history of dysmenorrhea was associated with subsequent endometriosis (odds ratio, 2.6; 95% confidence interval, 1.1-6.2). Despite a suggestive trend, shorter menstrual cycle length was not associated with endometriosis. Duration of natural menstruation and heaviness of flow were not associated with subsequent risk of endometriosis; neither was the reported type of sanitary protection used nor history of sexual intercourse during menstruation. CONCLUSION: There is a decreased risk of endometriosis with late age at menarche and an increased risk in women who report an early history of dysmenorrhea.