Background The presence of a suspicious ovarian cyst with elevated cancer antigen 125 level in a woman of reproductive age poses a serious therapeutic dilemma. Mature cystic teratomas and mucinous cystadenomas may also cause an increase in cancer antigen 125. Case presentation A 43-year-old Sinhalese woman with a history of anovulatory subfertility for 5 years presented with heavy menstrual bleeding and secondary dysmenorrhea of 6 months’ duration. Imaging (pelvic ultrasound and computed tomography of the abdomen and pelvis) revealed a hemorrhagic cyst (6 × 4 cm) on the right side and a multilocular cyst with solid areas (10 × 7 cm) on the left side. Her cancer antigen 125 level was 2715 U/ml. Following a multidisciplinary team meeting, a fertility-sparing staging laparotomy was performed, which included right cystectomy, left oophorectomy, infracolic omentectomy, and peritoneal washings. Histology revealed a mucinous cystadenoma of the right ovary and a mature cystic teratoma on the left ovary. No malignant cells were observed in peritoneal washings. The patient’s cancer antigen 125 level dropped to 74.8 U/ml 1 month after surgery. Conclusion Rarely, teratomas and mucinous cystadenomas may also give rise to an extremely high cancer antigen 125 level. The risk of malignancy index and risk of malignancy algorithm may both be misleading in these instances. Therefore, multidisciplinary input, fertility-sparing surgery, and follow-up are paramount to achieve optimal treatment and patient satisfaction.

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