Unilocular-solid ovarian cysts are a rare but challenging pathology in young women, with a desire to spare their fertility. In these cases, the risks of borderline and invasive disease are around 10 and 20%, respectively. No ultrasound rule has yet demonstrated the ability to discriminate with high accuracy, a borderline tumor from a benign tumor or 'invasive tumor'. The aim of this study was to assess the predictive performance of different ultrasound parameters in differentiating benign and borderline tumors versus invasive malignant tumors in premenopausal patients with unilocular-solid ovarian masses. Women aged ≤ 50 years with unilocular-solid adnexal masses with a maximum diameter ≤ 10 cm, undergoing surgery in our department within 3 months from ultrasound examination, were included in this retrospective study. A standardized ultrasound examination technique and predefined definitions of ultrasound characteristics were used. The results of ultrasound examination using gray scale and color Doppler were compared with the histological examination of the respective surgical specimens. The study included 51 patients. On histological examination, 36 (70%) lesions were classified as benign, 10 (20%) as borderline ovarian tumors and 5 (10%) as invasively malignant tumors. In receiver-operating characteristic curve analysis, the best cut-off for the largest solid component with regard to discriminating non-invasive (benign or borderline) from invasive tumors was 14 mm. A largest solid component >14 mm, the presence of papillation blood flow and the combination of the two parameters provided a sensitivity of 100% and a specificity of 63, 63 and 80%, respectively. Transvaginal ultrasound examination seems to be able to discriminate between invasive and non-invasive tumors in the premenopausal patients with unilocular-solid adnexal masses. Because of the retrospective nature of the study, further prospective clinical trials are needed to confirm the accuracy of the selected sonographic parameters in discriminating the invasive and non-invasive adnexal tumors.