In patients with cervical carcinoma, precise knowledge of parametrial tumor extension affects the therapeutic decision between surgery and radiation therapy. The purpose of this prospective study was to determine the efficacy of MR imaging in detecting the presence or absence of parametrial invasion in patients with cervical cancer thought clinically to be confined to the cervix. Twenty-five consecutive patients were included in the study. All patients underwent radical hysterectomy or total abdominal hysterectomy and had detailed histologic evaluation of the parametrium. Ten had pathologic evidence of parametrial invasion; in the remaining 15, no parametrial invasion was identified pathologically. MR findings were compared with pathologic findings in all cases. For determining parametrial involvement, MR imaging had an accuracy of 88%, a sensitivity of 100%, and a specificity of 80%. Our results suggest that MR imaging is a reliable means of assessing parametrial invasion by cervical cancer.