External validation of the diagnostic performance of the modified International Ovarian Tumour Analysis (IOTA) benign simple descriptors versus IOTA ADNEX in the classification of benign ovarian tumours. Ambispective diagnostic accuracy study based on ultrasound data collected at a university hospital with a gynecologic oncology unit between 2012 and 2022. Two IOTA-certified level III sonographers classified adnexal masses using modified benign descriptors (MBD) and the IOTA ADNEX. The main outcome was to compare the diagnostic accuracy percentage for each of the classification systems, based on all of the masses that met the MBD criteria. Histopathology was used as the diagnostic reference standard. Statistical differences were established from (p < 0.05). 534 patients with 547 adnexal masses met the criteria to enter the study. For MBD, 97.8% (535/547) were benign masses by histopathology. 2.2% (12/547) corresponded to false negatives, of which 58% (7/12) were Borderline tumours, 33% (4/12) endometrioid adenocarcinoma and 9% (1/12) Stage II-IV malignant ovarian tumour. For IOTA ADNEX 98.7% (540/547) were benign masses by histopathology. 1.2% (5/547) corresponded to false negatives, of which 80% (4/5) were borderline tumours and 20% (1/5) Stage II-IV malignant ovarian tumour. No statistically significant differences were found between the diagnostic accuracy percentages of both classification systems (p < 0.05). There are no statistically significant differences between MBD and IOTA ADNEX for the discrimination of benign tumours.