We described early features of MRI diffusion weighted images (DWIs) in three patients with lateral medullary infarction. Patient 1. A 65-year-old woman who complained of vertigo was admitted. Then, DWIs showed a high signal intensity in the right lateral medulla, though T2 weighted images (T2WIs) did not show any abnormalities. On the next day, when the typical symptoms and signs of lateral medullary infarction appeared, the lesion was recognized on both DWIs and T2WIs. Patient 2. A 48-year-old man was admitted 9 hours after he had dizziness, nausea, and repeated vomitings. When a diagnosis of Wallenberg syndrome was made on the neurological examination, MRI DWIs demonstrated a high signal intensity in the right lateral medulla. The lesion became apparent on T2WIs 82 hours after the onset. Patient 3. A 71-year-old man was admitted for nausea, dizziness, and repeated vomitings. A diagnosis of Wallenberg syndrome was made fifty eight hours after the onset, a lesion with high signal intensity in the right lateral medulla was evident on DWIs, but it was faint on T2WIs. We concluded that DWIs is useful in early diagnosis of lateral medullary infarction presenting Wallenberg syndrome.