PURPOSE: To differentiate endometrial cysts from hemorrhagic cysts on the basis of MR findings MATERIALS AND METHODS: The MR findings of twelve patients with endometrial cysts(15 cases) and of nine patients withhemorrhagic cyst(12 cases) were retrospectively evaluated. Fourteen patients were surgically corfirmed and sevenwith hemorrhagic cysts were clinically diagnosed by resolution of the cysts during ultrasound follow up. Sixteenpatients underwent MR imaging using a 1.5T system(Magnetom Vision, Siemens, Germany), and for five patients a 2.0Tsystem(Spectro 2000, Goldstar, Korea) was used. MR images were retrospectively evaluated with respect to size andsignal intensity of the cyst, uni/multilocularity, shading, the hematocrit effect, clot.., fluid-fluid level andseptum, and thickness, signal intensity and enhancement of the cyst wall. RESULT: Eleven(73.3%) endometrial cystswere multilocular, but all hemorrhagic cysts were unilocular. The signal was hyperintense on both T1WI and T2WI inten(66.7%) endometrial cysts and seven(58.3%) hemorrhagic cysts. Shading was found in five(33.3%) and one(8.3%),respectively; the hematocrit effect in two(13.3%) and five(41.7%) respectively, clot in two of each type(13.3%,16.7%), and fluid-fluid level in only one hemorrhagic cyst. Septum was found only in endometrial cysts(five cases,33.3%); its signal intensity on both T1WI and T2WI was low, and on Gd-enhanced images was not enhanced. The cystwall was thick in five of each type(33.3%, 41.7%); its signal intensity was low on both T1WI and T2WI, and notenhanced on Gd-enhanced images. In eight hemorrhagic cysts, however, the cyst wall was iso to high in signalintensity on both T1WI & T2WI, and was enhanced on Gd-enhanced images. The prevalence of uni/ multilocularity,septum, and signal intensity and the presence of enhancement of the cyst wall were significantly different between the two groups(p<0.005). CONCLUSION: Uni/ multilocularity, septum, and signal intensity and enhancement of thecyst wall were useful for the differentiation of endometrial from hemorrhagic cysts. Signal intensity of the cyst,shading, the hematocrit effect and clot were not helpful.