Abstract Objective. Risk of malignancy index (RMI), based on a serum cancer antigen 125 level, ultrasound findings and menopausal status, is used to discriminate ovarian cancer from benign pelvic mass. In Denmark, patients with pelvic mass and RMI =200 are referred to tertiary gynecologic oncology centers according to the national guidelines for ovarian cancer treatment. The guidelines include recalculation of RMI at the tertiary center and, if indicated, positron emission tomography/computed tomography and fast-track surgery by specialists in cancer surgery. The aim of this study was to validate the use of RMI =200 as a tool for preoperative identification of ovarian cancer at a tertiary center. Design. Prospective observational study. Setting. A tertiary center in Copenhagen, Denmark. Population. One thousand one hundred and fifty-nine women with pelvic mass. Methods. The RMI was calculated after ultrasound examination and blood sampling for serum cancer antigen 125 analysis within two weeks before surgery. Main outcome measures. Sensitivity, specificity, positive and negative predictive values were calculated to evaluate the ability of RMI to distinguish between ovarian cancer and benign pelvic mass. Results. There were 778 women diagnosed with benign pelvic mass, while 251 had ovarian cancer and 74 had borderline ovarian tumor. Fifty-six women were diagnosed with other forms of cancer. Sensitivity and specificity for ovarian cancer vs. benign pelvic mass for RMI =200 were 92 and 82%, respectively. Corresponding positive and negative predictive values were 62 and 97%. Conclusions. Risk of malignancy index =200 is a reliable tool for identifying patients with ovarian cancer pelvic masses at a tertiary centre to select patients for further preoperative examinations.