We report two young patients of Wallenberg syndrome who presented with acute vertigo as the initial symptom. Neither Case 1, an 8-year-old girl, nor Case 2, a 29-year-old man, showed any abnormal findings on the initial magnetic resonance imaging (MRI) on day 1. In addition to nystagmus, several neurological signs became positive during the clinical course. Finally, the diagnosis of Wallenberg syndrome was made in both cases based on the detection of a lateral medullary infarction in a follow-up MRI conducted on day 4. Stroke should be considered in the differential diagnosis of acute vertigo, even in young patients without risk factors for cerebrovascular disorders, such as hypertension, diabetes and/or hyperlipidemia. Since MRI may show no abnormalities in the very early stages of stroke, neurological examinations are useful to establish the diagnosis in cases presenting with vertigo induced by brain stem or cerebellar infarction.