The differential diagnosis between macrocystic serous cystoadenoma and mucinous cystoadenoma is very difficult but is highly important due to the latter's potential for malignant transformation. We present a case of unilocular macrocystic serous cystoadenoma. Due to the impossibility of obtaining an accurate preoperative diagnosis, all macrocystic unilocular lesions, in an appropriate clinical context, must be resected.