Borderline ovarian tumours are an intermediate entity between benign and malignant epithelial ovarian neoplasms. The authors present the case of a 25-year-old patient, with a history of fertility-preserving right salpingo-oophorectomy due to an ovarian serous papillary cystadenocarcinoma six years ago. The patient recently presented with a cyst in the left ovary which raised concerns for recurrent disease. The patient underwent a left ovarian cystectomy and the histopathological examination concluded in serous borderline tumour. The patient is followed up regularly, so that a possible relapse will be diagnosed in time. In conclusion, a borderline lesion in the remaining ovary, after ovarian conservative surgery, is a challenge that could represent either a de novo emergence of borderline tumour or possibly the onset of a new ovarian cancer. The study of early molecular alterations could be a promising tool in the investigation of prognosis and management of these lesions.