Twenty-nine patients with ovarian malignant tumor were studied by means of magnetic resonance (MR) imaging. The findings were compared with surgical findings concerning tumor invasion to the bladder and to the uterus. The bladder invasion was evaluated with MRI and classified as follows: (1) Huge amount of fat, colon or small intestine between the tumor and the bladder. (2) Thin fat layer between the bladder and the tumor, (3) No fat layer, and the bladder was smoothly compressed by the tumor, (4) No fat layer with local deformity of the bladder caused by the tumor existed, or tumor protruded into the bladder. In group (4), 4 of 5 cases were proved to have bladder invasion. However in group (3), half of the 14 cases showed bladder invasion and the others showed no invasion. Criteria used in MRI analysis of uterine invasion was as follows: (1) Fat layer between the uterus and the tumor, (2) Fat layer between the tumor and the bladder, and the uterine contour was indistinct, (3) No fat layer and deformity of the uterus was apparent. In group (3), many cases showed uterine invasion, but 4 cases out of 12 cases did not. In the group (2), 3 out of 11 cases showed uterine invasion. The diagnostic ability of uterine invasion was not as accurate as in cases of the bladder invasion. The MRI sagittal plane can directly show the relationship between the uterus, the bladder and tumor, so that diagnosis of tumor invasion by MRI is superior to CT. However MRI could not differentiate between adhesion and invasion.