Endometrioid borderline tumours (EBTs) of the ovary are uncommon tumours of low malignant potential. They consist of atypical endometrioid cells lacking destructive stromal invasion. As the prognosis of EBT is excellent, excessive treatment should be avoided and preoperative diagnosis is important. Here we report four cases of ovarian EBTs along with imaging findings and a review of literature. The average patient age was 52 years. They presented with abdominal discomfort or abnormal vaginal bleeding. The final diagnoses for all four cases were EBT Stage IA with endometriosis. Pathologically, one case was an adenofibromatous type tumour, and three cases were intracystic type tumours. Two patients had concurrent endometrial cancer. MRI of the tumours showed enhanced solid components. The intracystic type tumours presented a dendritic structure in the cyst; fine papillary branches surrounded a low-signal trunk on T 2 weighted imaging. Positron emission tomography demonstrated marked fluorodeoxyglucose uptake in the solid component. One case with MRI 3 years before diagnosis indicated that the tumour arose in ovarian endometriotic cyst. EBT cases were difficult to distinguish from malignant ovarian tumours preoperatively. Intraoperative frozen section analysis may aid to determine treatment. Prognoses were excellent. Care should be taken for co-existing endometrial cancer.