A 65-years-old female was hospitalized 24 h after experiencing the sudden onset of subjective reduction in visual acuity and hypersomnia. On admission to the neurological ward, she presented isolated downgaze palsy. A Magnetic Resonance Imaging of the brain disclosed a discrete, ovalar hyperintensity involving the left paramedian thalamic-mesencephalon junction. The lesion was consistent with infarction. Isolated downgaze palsy has been described in thrombosis of Artery of Percheron leading to infarction of bilateral paramedian thalami along with structures from the mesencephalic-diencephalic junction such as the Medial Longitudinal Fasciculus (riMLF). While neurons from the riMLF controlling upward vertical saccades project to either ipsilateral and contralateral oculomotor nuclear complexes, those involved in regulating downgaze descend ipsilaterally in the brain stem. Isolated downgaze palsy has an extreme localizer value to the diencephalic-mesencephalon junction and can arise from a unilateral lesion.