Objective: To determine whether accuracy of visual diagnosis of endometriosis at laparoscopy is determined by stage of disease. Design: Prospective longitudinal cohort study (Canadian Task Force classification II-2). Setting: Tertiary referral centers in three Australian states. Patients: Of 1439 biopsy specimens, endometriosis was proved in at least one specimen in 431 patients. Interventions: Laparoscopy with visual diagnosis and staging of endometriosis followed by histopathologic analysis and confirmation. Operations were performed by five experienced laparoscopic gynecologists. Measurements and Main Results: Histopathologic confirmation of visual diagnosis of endometriosis adjusted for significant covariates. Endometriosis was accurately diagnosed in 49.7 of American Society for Reproductive Medicine (ASRM) stageI, which was significantly less accurate than for other stages of endometriosis. Deep endometriosis was more likely to be diagnosed accurately than superficial endometriosis (adjusted odds ratio, 2.51; 95 confidence interval, 1.50-4.18; p<.01). Lesion volume was also predictive, with larger lesions diagnosed more accurately than smaller lesions. In general, lesion site did not greatly influence accuracy except for superficial ovarian lesions, which were more likely to be incorrectly diagnosed visually as endometriosis (adjusted odds ratio, 0.16; 95 confidence interval, 0.06-0.41; p <.01). There was no statistically significant difference in accuracy between the gynecologic surgeons. Conclusion: The accuracy of visual diagnosis of endometriosis was substantially influenced by American Society of Reproductive Medicine stage, the depth and volume of the lesion, and to a lesser extent the location of the lesion.