Abstract
•A 56-year-old woman developed two seromucinous borderline tumors 26 years apart.•The second cyst was diagnosed as a seromucinous borderline tumor associated with pelvic endometriosis.•The first ovarian cancer was re-diagnosed as an ovarian seromucinous borderline tumor after a pathological slide review.•Seromucinous borderline tumors can re-occur several years after post-treatment estrogen replacement therapy.•Post-treatment estrogen replacement therapy for seromucinous borderline tumors should be provided carefully.