Objectives Recently we conducted a study in which we found an increased association of ovarian cancer in women with endometriosis. Analyses showed that the cohort included both women with endometriosis externa and adenomyosis. Therefore, in the present study we assessed the association between endometriosis and/or adenomyosis and ovarian cancer.

Methods We identified all women with histological proven endometriosis (51,544 women) and/or adenomyosis (85,015 women) from the Dutch pathology database (1990–2015) and matched them with women with a benign dermal nevus (132,654 women). Histology results for ovarian cancer were retrieved. We estimated crude and age-adjusted incidence rate ratios (IRR) for ovarian cancer.

Results We found 1,017 (2.0%), 1,284 (1.5%) and 471 (0.4%) ovarian cancer cases in the endometriosis, adenomyosis and nevus cohort, respectively. The age-adjusted IRRs were 19.75 (95%CI 16.70–23.35) in the endometriosis cohort and 5.93 (95%CI 4.91–7.16) in the adenomyosis cohort (table 1). The highest IRRs were found for endometrioid and clear cell ovarian cancer subtypes (table 1). Excluding the first year of follow-up did not result in a significant IRR for ovarian cancer overall but resulted in a statistically significant IRRs for clear cell and endometrioid ovarian cancer (table 1).

Conclusions We found an increased ovarian cancer incidence in both histological proven endometriosis and adenomyosis. This increased incidence was largest for endometriosis. Excluding the first year of follow-up resulted in an increased incidence for endometrioid ovarian cancer in both cohorts and clear cell ovarian cancer in the endometriosis cohort.