The triad of Horner's syndrome, ataxia and sensory disturbances clinically identifies patients with lateral medullary infarct (LMI). However, dermatomal temperature and pain sensory loss have rarely been reported in patients with LMI. We describe the case of a patient with LMI presenting solely with temperature and pain sensory disorder below the C5 sensory level. Magnetic resonance imaging findings demonstrated a small superficial infarction in the right lateral medulla oblongata, indicating that the sensory deficit was caused by a restricted infarction of the lateral spinothalamic tract. We emphasize the importance of adding LMI to a differential diagnosis of acute dermatomal sensory disturbance without other major symptoms of LMI, such as cranial nerve abnormalities, ataxia and Horner's syndrome.