PURPOSE: To characterize the magnetic resonance (MR) findings of struma ovarii, a rare ovarian tumor composedsolely or predominantly of thyroid tissue or the tumor in which hyperthyroidism results from the ovarian thyroidtissue. MATERIALS AND METHODS: In seven patients, MR images (T1-weighted with or without gadolinium enhance-mentand T2-weighted) were obtained in the axial, coronal, and sagittal planes using 1.5-T MR units, and in three ofseven patients the fat saturation technique were performed. MR findings were retrospectively evaluat-ed for thesite, size, components, signal intensity, presence and degree of contrast enhancement, and associat-ed findings.These and pathologic findings were compared. RESULTS: MR images showed unilateral complex masses composed ofmultiple cysts and some solid components, corresponding pathologically to thyroid follicles containing colloid andthe stroma with abundant blood vessels and fibrous tissue. Cystic portions of the tumors had variable signalintensities on T1-weighted images, and high signal intensities on T2-weighted images. Some hyperintense cysticareas seen on T1-weighted images were due to hemorrhage (n=1) and colloid components (n=3). Solid portions of thetumors were isointense relative to adjacent muscles (with intense contrast enhancement), as seen on T1-weightedimages, and iso- or hyperintense on T2-weighted images. CONCLUSION: Struma ovarii has a characteristic MRappearance of a complex mass composed of multiple cysts and intensely enhanced solid components.