Second fertility preservation surgery for early relapse of seromucinous borderline ovarian tumors.

•The initial diagnosis was endometriosis; however, the postoperative diagnosis was a seromucinous borderline ovarian tumor.•Women of reproductive age experienced an early relapse of seromucinous borderline ovarian tumor after surgery.•Transvaginal ultrasound was useful for early diagnosis of relapse.•A second fertility preservation surgery was performed, and fertility treatment without relapse was ongoing.