The authors report the case of a patient with an ischemic lesion in the left midbrain. The patient presented with paresis of left inferior rectus, pupil, right superior rectus, convergence and transiently, of the left medial rectus. A lesion in the left dorsal midbrain close to the oculomotor nuclear complex, selectively involving the fascicles innervating the above muscles, is proposed. Fine magnetic resonance sections showed a consistent lesion in the left paramedian dorsal midbrain. A detailed, three-dimensional, schematic computer model of the oculomotor nucleus and fascicles was constructed. Using this model, the authors topographically validate the putative site of the lesion. The medial rectus subnucleus is divided into three subgroups, A, B, and C. Subgroup C is thought to be the site of the majority of neurons controlling convergence. In the above model, the putative lesion is closer to subgroup A than to C; this suggests that subgroup A, rather than subgroup C, may have a higher concentration of neurons involved in convergence.