Objective To evaluate the ability of a risk of malignancy index (RMI), based on a serum CA125 level, ultrasound findings and menopausal status, to discriminate a benign from a malignant pelvic mass and to discriminate early stage (Figo Stage I) from Stages II, III and IV of ovarian cancer. Design A prospective study. Setting Department of Gynaecology, Trondheim University Hospital, Trondheim, Norway. Participants One hundred and seventy‐three women, 30 years or older, consecutively admitted between February 1992 and February 1994 for primary laparotomy of a pelvic mass. Main outcome measures The sensitivity, specificity and positive predictive value of serum CA125 level, ultrasound findings and menopausal status, separately and combined into the RMI, to diagnose ovarian cancer. Results The RMI was more accurate than any individual criterion in diagnosing cancer. Using a RMI cut‐off level of 200 to indicate malignancy, the RMI derived from this dataset gave a sensitivity of 80%, specificity of 92% and positive predictive value of 83%. Applying RMI criteria developed by others, the following test performance was found: sensitivity 71%, specificity 96% and positive predictive value 89%. For the Stages II, III and IV of ovarian cancer the sensitivity increased to approximately 90% without any substantial loss in specificity. Conclusions The risk of malignancy index is able to correctly discriminate between malignant and benign pelvic masses. It is a scoring system which can be introduced easily into clinical practice to facilitate the selection of patients for primary surgery at an oncological unit.

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