Leiomyoma and adenomyosis of the uterus are the most common gynecologic disorders in an enlarged uterus. The characteristic US and MR findings in differentiation between both lesions were prospectively evaluated in 30 patients. Of 30 patients, 15 were leiomyomas, 6 were adenomyosises, 8 were leiomyomas and adenomyosises, and 1 was a normal pregnancy, histologically. The total number of leiomyom nodules were 49 while adenomyosises were 14 (9 diffuse and 5 focal). Among 49 myomas nodules, 36 were correctly diagnosed by sonography. The characteristic US findings of uterine leiomyoma were well defined nodules (36), hypoechoic peripheral rim (16), and whorl-like internal echoes (13). Forty four of the 49 myoma nodules were correctly diagnosed by MRI. The characteristic MR findings of myoma were well defined nodules (43), peripheral low signal intensity rim on T1WI (13) and T2WI (9), and peripheral high signal intensity rim on T2WI (5). Among 14 adenomyosises, 9 were correctly diagnosed by sonography. The characteristic US findings of adenomyosis were diffuse uterine hypertrophy more than 5.5cm in AP diameter with endometrial displacement and no significant echo change in myometrium All 14 adenomyosis as were correctly diagnosed from MRI. On T2WI, adenomyosis appeared as ill defined localized or diffuse thickening of the junctional zone more than 1cm in thickness. It was our conclusion that to differentiate between leiomyoma and adenomyosis focused on should be the detection of existence of nodule in leiomyoma, the primary sign, not on the secondary indirect sign.