Objectives: To estimate intra-observer repeatability and inter-observer agreement in 1) describing adnexal masses using the International Ovarian Tumor Analysis (IOTA) terms and definitions, 2) the risk of malignancy calculated using IOTA logistic regression models LR1 and LR2, 3) the diagnosis made on the basis of subjective assessment of ultrasound images. Methods: One hundred and three adnexal masses were examined with transvaginal gray scale and power Doppler ultrasound using a GE Voluson 730 Expert system. Three-dimensional ultrasound volumes of the mass were saved. After 12-18 months the volumes were analyzed twice 1-6 months apart by each of two independent experienced sonologists who used the IOTA terms and definitions to describe the masses. The risk of malignancy was calculated using LR1 and LR2. The sonologists also classified the masses as benign or malignant using subjective assessment. Results: Eighty-four masses were benign, eight borderline and 11 invasively malignant. There was substantial variability within and between observers in the results of measurements included in LR1 and LR2 and some variability also when assessing categorical variables included in the models (agreement 51-100%, Kappa 0.42 -1.00). This resulted in substantial variability in the calculated risk of malignancy, the limits of agreement indicating that the calculated risk of malignancy could vary by a factor of five to twenty within and between observers. The reliability of the calculated risk of malignancy was moderate (LR1) or poor (LR2) when the calculated risk of malignancy was > 10% (intra-class correlation coefficients varying from 0.21 to 0.73). Inter-observer agreement when classifying tumors as benign or malignant using the predetermined risk of malignancy cut-off of 10% was fair to good (agreement 85%, Kappa 0.61 for LR1; agreement 81%, Kappa 0.52 for LR2). Intra- and inter-observer agreements for subjective assessment were 96%, 96% and 96% with Kappa values of 0.89, 0.87 and 0.88. Conclusions: Intra- and inter-observer agreement in classifying tumors as benign or malignant using the risk of malignancy cut off of 10% for LR1 and LR2 was fair or good, while the reproducibility of subjective assessment was excellent. The reliability of calculated risks > 10% was poor, and calculated risk > 10% cannot be used to discriminate between individuals at different risk. These results cannot be extrapolated to real-time ultrasound examinations. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.