ObjectivesWomen with endometriosis carry an increased risk for ovarian clear cell adenocarcinomas (CCCs). Clear cell adenocarcinoma may develop from endometriosis lesions. Few studies have compared clinical and prognostic factors and overall survival in patients diagnosed as having CCC according to endometriosis status.MethodsPopulation-based prospectively collected data on CCC with coexisting pelvic (including ovarian; n = 80) and ovarian (n = 46) endometriosis or without endometriosis (n = 95) were obtained through the Danish Gynecological Cancer Database. χ² Test, independent-samples t test, logistic regression, Kaplan-Meier test, and Cox regression were used. Statistical tests were 2 sided. P values less than 0.05 were considered statistically significant.ResultsPatients with CCC and pelvic or ovarian endometriosis were significantly younger than CCC patients without endometriosis, and a higher proportion of them were nulliparous (28% and 31% vs 17% (P = 0.07 and P = 0.09). Accordingly, a significantly higher proportion of women without endometriosis had given birth to more than 1 child. Interestingly, a significantly higher proportion of patients with ovarian endometriosis had pure CCCs (97.8% vs 82.1%; P = 0.001) as compared with patients without endometriosis. Overall survival was poorer among CCC patients with concomitant ovarian endometriosis (hazard ratio, 2.56 [95% confidence interval, 1.29–5.02], in the multivariate analysis.ConclusionsAge at CCC diagnosis and parity as well as histology differ between CCC patients with and without concomitant endometriosis. Furthermore, CCC patients with concomitant ovarian endometriosis have a poorer prognosis compared with endometriosis-negative CCC patients. These differences warrant further research to determine whether CCCs with and without concomitant endometriosis develop through distinct pathogenic pathways.