BACKGROUND AND PURPOSE: The oculomotor nerve fascicles arise along its entire length and sweep ventrally to exit the midbrain at the medial edge of the crus cerebri. A rostro-caudal topography among the fascicular fibers is relatively well established. There are, however, some controversies whether medio-lateral topography also exists. METHODS: We retrospectively reviewed the clinical records and MRI of the 8 patients showing isolated oculomotor nerve palsy due to midbrain infarction. Brain MRI was performed using a 1.5-T magnet with 2mm thickness and 0.1 mm slice interval. The anterior-posterior axis(X) was defined as the midline crossing the center of the cerebral aqueduct and the medio-lateral axis(Y) as the line crossing the same point. For rostro-caudal measurement, the intercommissural line was used as base line of the Z axis. The location of the lesions was defined by measuring actual distance of the margins of the lesions in millimeter from each axis; anterior, right, and caudal direction was defined as positive values in X, Y and Z coordinates, respectively. RESULTS: The mean values and range of the X, Y and Z are as follows: X=7.56+/-4.34, 1< or =X< or =15; Y=3.43+/-1.37, 0< or =Y< or =6; Z=6.51+/-3.91, 0< or =Z< or =12.5. CONCLUSIONS: The distribution of all the MRI lesions was 0< or =|Y|< or =6 (mm), 0< or =|Z|< or =12.5 (mm) in mediolateral and rostrocaudal direction respectively, which is almost the same as the previously reported divergent range of the oculomotor fascicles in midbrain tegmentum. We suggest that our method of three dimensional measurements of the MRI lesion in midbrain tegmentum could be a useful tool for the study of oculomotor fascicular arrangement.