Magnetic resonance (MR) imaging characteristics of adenomyosis were studied in eight women (aged 37-49 years) who underwent hysterectomy, and detailed radiologic/pathologic correlation was conducted in all cases. Adenomyosis produced diffuse and smooth uterine enlargement. The extent of the lesion was clearly identified on images obtained with long repetition time and long echo time; a diffuse, low-intensity area accompanied by tiny high-intensity spots was seen subjacent to the endometrium. The area appeared as a localized or diffuse thickening of the junctional zone because it was often isointense with this zone. Pathologic examination confirmed that the extent of adenomyosis correlated well with the low-intensity region on MR images and that both hemorrhagic areas and nonbleeding endometrial tissue corresponded to the high-intensity spots. The lesion consisted of distorted and compacted smooth muscle cells, but microscopic studies failed to explain the definitive difference in intensity between areas of adenomyosis and myometrium.