Background: Estimating the risk of malignancy is essential in the management of adnexal masses. An accurate differential diagnosis between benign and malignant masses will reduce morbidity and costs due to unnecessary operations, and will improve referral to a gynecologic oncologist for specialized cancer care, which improves outcome and overall survival. The Risk of Malignancy Index is currently the most commonly used method in clinical practice, but has a relatively low diagnostic accuracy (sensitivity 75-80 % and specificity 85-90 %). Recent reports show that other methods, such as simple ultrasound-based rules, subjective assessment and (Diffusion Weighted) Magnetic Resonance Imaging might be superior to the RMI in the pre-operative differentiation of adnexal masses. Methods/Design: A prospective multicenter cohort study will be performed in the south of The Netherlands. A total of 270 women diagnosed with at least one pelvic mass that is suspected to be of ovarian origin who will undergo surgery, will be enrolled. We will apply the Risk of Malignancy Index with a cut-off value of 200 and a two-step triage test consisting of simple ultrasound-based rules supplemented -if necessary-with either subjective assessment by an expert sonographer or Magnetic Resonance Imaging with diffusion weighted sequences, to characterize the adnexal masses. The histological diagnosis will be the reference standard. Diagnostic performances will be expressed as sensitivity, specificity, positive and negative predictive values and likelihood ratios. Discussion: We hypothesize that this two-step triage test, including the simple ultrasound-based rules, will have better diagnostic accuracy than the Risk of Malignancy Index and therefore will improve the management of women with adnexal masses. Furthermore, we expect this two-step test to be more cost-effective. If the hypothesis is confirmed, the results of this study could have major effects on current guidelines and implementation of the triage test in daily clinical practice ...