OBJECTIVE: We investigated the clinical value of using preoperative differential white blood cell (WBC) count to predict the potential for malignancy of adnexal masses in laparoscopic surgery. METHODS: The electronic medical records of 1325 patients who underwent laparoscopic surgery for adnexal masses between July 2005 and December 2008 were analyzed retrospectively. RESULTS: Of 1325 patients, 30 (2.3%) had adnexal masses with malignant potential. Analysis of differential WBC count, neutrophil to lymphocyte ratio (NLR), neutrophil to monocyte ratio (NMR), serum CA 125, mass size showed that only cyst size was significantly different between patients with potentially malignant adnexal masses, those with benign disease (averages of 9.45 cm vs. 6.23 cm, p=0.001). Further analysis was performed using a combination of various markers and multiplication of cyst size and NMR yielded the highest area under the curve, at 0.711(95% confidential interval 0.619~0.806, p<0.001), with a sensitivity and specificity of 86.7% and 48.3% respectively, at a cut off value of 67.23. These values were also significantly different between patients with potentially malignant adnexal masses, and dermoid cyst or endometrioma (p=0.038 and 0.002 respectively, by analysis of variance, post hoc test). CONCLUSION: Preoperative measurement of NMR in conjunction with cyst size may be used as a simple, non invasive marker for predicting the malignant potential of adnexal masses before laparoscopic surgery.