The aim of this study was to perform an external validation of IOTA simple rules to differentiate between benign and malignant adnexal masses. This is a retrospective diagnostic accuracy study of data collected prospectively from patients with adnexal pathology who underwent transvaginal ultrasound, performed by gynecologists and radiologists, experts in ultrasound, prior to surgery. This study was conducted in a tertiary university oncology hospital. Data were collected between January of 2012 and December of 2018. We used the IOTA simple rules to classify the masses. Definitive pathology after tumour surgical removal was the reference standard used in this study. Sensitivity, specificity, positive (PPV) and negative predictive values (NPV) were calculated. A total of 587 patients underwent an ultrasound examination in our institution. 24 women were excluded for they were not operated. Thus, a total of 563 women were ultimately included (415 premenopausal and 148 postmenopausal). 35 women had bilateral masses, so 608 masses were evaluated. Final pathology revealed 65 malignant and 543 benign tumors. 90 (15%) of the masses could not be classified using SR. Sensitivity, specificity, PPV and NPV values for masses that could be classified was 83%, 97%, 74% and 99%, respectively. When considering those masses classified as inconclusive as malignant, sensitivity, specificity, PPV and NPV values was 89%, 85%, 42% and 98%, respectively. In this external retrospective validation study IOTA simple rules showed good diagnostic performance. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.