In order to improve the accuracy of magnetic resonance (MR) imaging assessment of myometrial invasion by endometrial cancer, the usefulness of a new diagnostic parameter, the endometriummyometrium (EM) ratio has been evaluated. EM ratio is the proportion of the widest length of endometrium to the length of myometrium measured at the same line, this being vertical to the parallel of the long axis of the uterine body in the sagittal plane of the MR images. Myometrial invasion was defined as a value of the EM ratio > 1, and the tumor was limited to the endometrium for values < 1. In 25 consecutive patients, both the EM ratio-based assessment and the well-established junctional zone-based asessment with T2-weighted MR imaging and enhanced MR imaging with gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) were compared with the results from pathological examinations of postoperative specimens. In identifying moymetrial invasion by endometrial cancer, the sensitivity of the EM ratio-based assessment was better than that of the junctional zone-based assessment. The overall sensitivity of the former was 96% in both the T2-weighted and enhanced MR imaging with Cd-OTPA, whereas that of the latter was 84% in the T2-weighted MR imaging and 72% (P<0.05) in the enhanced MR imaging. The use of the EM ratio with MR imaging improves the ability to assess myometrial invasion by endometrial cancer. © 1993 Oxford University Press.