External validation of the diagnostic performance of the two-step modified International Ovarian Tumour Analysis (IOTA) benign simple descriptors and IOTA ADNEX. 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 simple descriptors (MBD) (first step). Unclassifiable masses in the first step were evaluated using the IOTA ADNEX model (second step). The main outcome was the classification of tumours as benign or malignant. Histopathology was used as the diagnostic reference standard. For analysis, the diagnostic performance of the two-step strategy was estimated. 871 patients with 880 adnexal masses were included in the study, of which 547 (62%) met the classification criteria for their evaluation based on MBD (first step) and 333 (38%) were assessed using IOTA ADNEX. 98% (535/547) of the masses classified according to the MBD, were benign by histopathology. The diagnostic performance of IOTA ADNEX (second step) had a sensibility of 92%, specificity of 67%, positive predictive value of 49%, negative predictive value of 96% and area under the curve of 0.88 (CI 95%, 0.83–0.92). MBD had the ability to correctly classify most of the evaluated masses. Regarding the two-step strategy, it had an appropriate diagnostic performance, which is why it is recommended for clinical use.