Objective: To retrospectively review the causes of categorization errors using O-RADS-MRI score and to determine the presumptive causes of these misclassifications.Methods: EURAD database was retrospectively queried to identify misclassified lesions. In this cohort, 1194 evaluable patients with 1502 pelvic masses (277 malignant/1225 benign lesions) underwent standardized MRI to characterize adnexal masses with histology or 2 years follow-up as reference standard. An expert radiologist reviewed cases with two junior radiologists and lesions termed misclassified if malignant lesion was scored≤ 3, a benign lesion was scored≥4, the site of origin was incorrect, or a non-adnexal mass was incorrectly categorized as benign or malignant.Results: There were 139/1502 (9.2%) misclassified masses in 116 women including 109 adnexal and 30 non-adnexal masses. False negative cases corresponded to 16 borderline or invasive malignant adnexal masses rated score≤3 (16/139,11.5%). False positive cases corresponded to 88 benign masses were rated score 4 (67/139,48.2%) or 5 (18/139,12.9%) or considered as suspicious non-adnexal lesions (3/139,2.2%). Misclassifications were only due to origin error in 12 adnexal masses (8 benign, 4 malignant) (8.6%,12/139) and 23 nonadnexal masses (18 benign, 5 malignant,16.5%,23/139) perceived respectively as non-adnexal and adnexal masses. Interpretive errors (n=104), failure to recognize technical insufficient exams (n=9) and perceptual errors (n=4) were found. Most interpretive were due to misinterpretation of solid tissue or incorrect assignment of mass origin. 84/139 cases were correctly reclassified by the readers with strict adherence to the score rules. Conclusion:Most errors were due to misinterpretation of solid tissue or incorrect assignment of mass origin..