Background and Purpose The initial diagnosis of medullary infarction can be challenging since CT and even MRI results in the very acute phase are often negative. Methods A retrospective, observer-blinded study of horizontal conjugate eye deviation was performed in 1) 50 consecutive patients [age 58±15 years (mean±SD), 74% male, National Institutes of Health Stroke Scale 2±1] with acute unilateral lateral medullary infarction as seen in MRI (infarction group), 2) 54 patients with transient brainstem symptoms [transient ischemic attack of brainstem (TIA) group; age 69±16 years, 59% male], and 3) 53 patients (age 59±20 years, 49% male) with diagnoses other than stroke (control group). Results Conjugate eye deviation was found in all patients in the infarction group [n=47 (94%) with ipsilesional deviation and n=3 (6%) with contralesional deviation] compared to 41% (n=22) in the brainstem TIA group and 15% (n=8) in the control group (p<0.0001). Within all groups mean deviation and range were similar for both sides (to the right vs. to the left side 26.6°±12.3 vs. 26.1°±12.3 in the infarction group, 10.5°±5.8 vs. 8.4°±6.3 in the brainstem TIA group and 4.5°±3.2 vs. 7.5°±3.2 in the control group). The extent of eye deviation was significantly greater in the infarction group (p<0.05). Conclusions All patients with MRI-demonstrated unilateral medullary infarction showed conjugate eye deviation. Therefore, conjugate eye deviation in patients with suspected acute lateral medullary infarction is a helpful sensitive sign for supporting the diagnosis, particularly if the deviation is >20°.