The differentiation of a uterine leiomyoma from other solid pelvic masses on sonography is usually straightforward. Occasionally, the sonographic appearance of a pedunculated uterine leiomyoma may simulate that of a solid adnexal mass. The purpose of this study was to determine if MR imaging adds specificity to the diagnosis of indeterminate solid pelvic masses visualized with sonography. Nineteen patients were imaged with MR after sonography revealed the presence of a solid pelvic mass adjacent to the uterus but could not be used to determine whether the mass was a leiomyoma or some other type of tumor. The diagnostic criteria for a leiomyoma on MR imaging included (1) the presence of a mass adjacent to the uterus and (2) a mass that was predominantly low signal intensity or isointense compared with normal myometrium on T1-weighted images and predominantly low signal intensity on T2-weighted images. In 11 of 13 patients, the masses that met these MR criteria for leiomyoma were proved to be uterine leiomyomas at surgery. Another mass that met the criteria was shown to be a leiomyoma in the broad ligament; the other was an ovarian fibroma. Of the six cases that did not meet the MR criteria for the diagnosis of leiomyoma, three were proved to be degenerated fibroids, one was squamous cell carcinoma of the cervix, and two were ovarian malignant tumors. Because leiomyomas often have an MR appearance that is distinct from that of solid pelvic malignant tumors, MR imaging can be useful for the diagnosis of some indeterminate solid pelvic masses.