The purpose of this study was to evaluate the clinical value of serum tumor markers in patients with ovarian mature cystic teratoma (MCT). We retrospectively evaluated 163 women who underwent surgery for MCT of the ovary between March 2003 and August 2007 and who provided preoperative blood samples for the measurement of CA 19-9 and CA 125. The rates of elevated serum CA 19-9 and CA 125 levels were 31.9% (52/163) and 13.5% (22/163), respectively. The rate of ovarian torsion was 12.9% (21/163). There were significant differences between the elevated CA 19-9 group and the normal CA 19-9 group in the diameters of the tumors and the rates of ovarian torsion. Elevated serum CA 19-9 levels correlated with larger tumor diameters and higher torsion rates. CA 19-9 may be a useful tool for the diagnosis of ovarian MCT. Elevated CA 19-9 levels appear to correlate with larger tumor diameters and higher rates of ovarian torsion.