We present a case of a 58-year-old menopausal woman referred to our hospital for the presence of large pelvic masses diagnosed by clinical examination and pelvic ultrasound. MRI examination showed voluminous bilateral capsulated multilocular ovarian cysts slightly hyperintense on T1-weighted images with thick septa and small papillary projections. CT scan confirmed the MRI findings. Among the ovarian tumor markers analyzed (CA125, HE4, and CA72.4), only Ca125 was slightly increased (48 U/ml). These data were suggestive of mucinous ovarian tumor. The patient underwent total hysterectomy with bilateral salpingo-oophorectomy, appendectomy, and multiple peritoneal biopsies. Pathological examination revealed bilateral borderline mucinous ovarian tumor with superficial atypical implants. Nine months later, the patient complained of left coxofemoral pain and underwent a PET/TC total body that suggested pubic bone metastases. Ovarian tumor markers were analyzed, and a second PET/TC was performed. CA125 was 252 U/ml, HE4 62 pM/L, and CA72.4 > 100 U/Ml. PET/TC was suggestive of peritoneal carcinosis. The patient was readmitted to the hospital. Clinical examination revealed small vaginal nodules. All nodules were excised. Microscopic analysis of all specimens revealed metastatic mucinous adenocarcinoma of intestinal type.The case shows that even a slight CA125 increase in the presence of a borderline ovarian tumor should not be overlooked since it can be indicative of a progressive disease. This case also highlights its additional diagnostic value when serum CA125 analysis is used in conjunction with MRI and CT imaging for the prognosis of mucinous borderline ovarian tumors (mBOTs).