PURPOSE: To determine the detection rate of MRI in the diagnosis of adenomyosis and ascertain the accuracy of MRI in assessing the extent and depth of the myometrial invasion. MATERIALS AND METHODS: By retrospective analysis of MR images of the pelvis in 65 pathologically proven cases of uterine adenomyosis, we investigated the detection rate and differences of in the detection rate when this was based on (a) the extent, and (b) the depth of myometrial invasion. The condition was classified as diffuse or focal according to the extent of invasion, and mild, moderate, or severe according to its depth. RESULTS: Pathologically, there were 35 cases of focal adenomyosis (53.8%), and 30 of diffuse adenomyosis (46.2%). Among patients with the focal variety of this condition, 12 cases were mild, 14 were mild, 14 were moderate, and nine were severe, while among those with the diffuse variety, two were mild, ten were moderate and 18 were severe. A total of 48 cases (73.8%) were detected on MR images; the detection rate of focal adenomyosis was 60.0% (21/35) and that of diffuse adenomyosis was 90.0% (27/30). The detection rates of mild, moderate, and severe adenomyosis were 42.9%(6/14), 79.2%(19/24), and 85.2% (23/27), respectively. Among the 48 cases detected on MR images, the pathologic and MR findings were consistent with regard to both the extent and depth of myometrial invasion in 26(54.2%). For diffuse adenomyosis, the consistency rate was higher than for the focal variety (81.5%, 22/27; compared with 19%, 4/21). The extent of myometrial invasion was correctly evaluated in 32 cases (66.7%); the consistency rate for the diffuse form was higher than for the focal form [96.3% (26/27), compared with 28.6% (6/21)]. In 42 cases (87.5%), the depth of invasion was correctly evaluated, though differences in this depth were not significant. CONCLUSION: MR imaging was a useful imaging modality in the diagnosis of uterine adenomyosis. With regard to the extant of mymetrial invasion, the detection rate was higher among cases of the diffuse form than of the focal form, and with regard to depth, the rate was higher among moderate and severe cases than among mild ones. These findings may be useful for preoperative diagnosis based on clinical symptoms, and provide important yardstick for decisions as to whether hysterectony should be replaced by an alternative therapy.