Objective To evaluate the ability of a risk of malignancy index (RMI), based on serum CA125 level, ultrasound findings and menopausal status, to discriminate benign from malignant pelvic mass. Methods One hundred and forty women with pelvic masses, at age 30 of years or more were admitted to the Peking Union Medical College Hospital between January 1998 and June 1999. The sensitivity, specificity and positive predictive value of serum CA125 level, ultrasound findings and the menopausal status in diagnosis of ovarian cancer were evaluated separately or combined into the RMI. Results RMI was more accurate than any individual criterion in diagnosing cancer. Using an RMI cutoff level of 200 to indicate malignancy, the RMI derived from this data set gave a sensitivity of 87.3%, a specificity of 84.4%, and a positive predictive value of 82.1%. Conclusions RMI is able to correctly discriminate malignant from benign pelvic mass. It can be introduced easily into clinical practice to facilitate the selection of patients for primary surgery. Objective To evaluate the ability of a risk of malignancy index (RMI), based on serum CA125 level, ultrasound findings and menopausal status, to discriminate benign from malignant pelvic mass. Methods One hundred and forty women with pelvic masses, at age 30 of years or more were admitted to the Peking Union Medical College Hospital between January 1998 and June 1999. The sensitivity, specificity and positive predictive value of serum CA125 level, ultrasound findings and the menopausal status in diagnosis of ovarian cancer were or will be separately or combined into the RMI Results RMI was more accurate than any individual criterion in diagnosing cancer. Using an RMI cutoff level of 200 to include malignancy, the RMI derived from this data set gave a sensitivity of 87.3%, a specificity of 84.4%, and a positive predictive value of 82.1%. Conclusions RMI is able to correctly discriminate malignant from benign pelvic mass. It can be incorporated easily into clinical practice to facili tate the selection of patients for primary surgery.