OBJECTIVE The aim of this study was to evaluate the role of the serum osteopontin (OPN) level as a biomarker for discriminating between malignant and benign ovarian tumors. Furthermore, comparisons with the diagnostic usefulness of the other tests were performed. METHODS The study included 114 consecutive women with ovarian tumors (82 benign and 32 malignant) who were referred to our division. RESULTS A cut-off level of 28.0 ng/mL for OPN showed a sensitivity of 71.87% and a specificity of 89.02%. The area under the receiver-operator curve (ROC) was 0.812. There were no differences in diagnostic utility between OPN and the other studied tests. OPN levels were lower in patients with endometriotic ovarian cysts than in those with other benign ovarian tumors (14.00 vs 19.50 ng/mL; P = 0.018). The difference between the median OPN level in patients with endometriotic cysts (14.0 ng/mL) and those with malignant tumors (40.85 ng/mL) was also statistically significant (P < 0.0001). The calculated OPN/CA-125 ratio was significantly different between patients with endometriotic cysts (median, 0.36; range, 0.05-2.89) and those with other benign tumors (median, 1.25; range, 0.05-5.70) (P = 0.0002). There was also a statistically significant difference in the median OPN/CA-125 ratio between patients with endometrial cysts (median, 0.36; range, 0.05-2.89) and those with malignant tumors (median, 0.12; range, 0.01-3.39) (P = 0.004). CONCLUSIONS The diagnostic utility of OPN is similar to that of ultrasonographic evaluation and CA-125 level assessment. Thus, OPN may be useful in differential diagnosis for less experienced ultrasonographers and is especially valuable for differential diagnosis of endometriotic cysts.