Descriptions in the literature of magnetic resonance imaging (MRI) findings in post-vaccination myelitis are scarce. Described here is a case of acute transverse myelitis that occurred after administration of an influenza vaccination. T1-weighted MRIs showed diffuse, fusiform spinal cord enlargement, extending from C-3 to rostral thoracic levels. Intramedullary lesions containing increased T2 signal were found in the areas of cord enlargement. The involvement on MRI was profound, extending far rostral to the level of the discrete clinical myelopathy. The lesions did not enhance after contrast administration. The patient had a complete long-term recovery of neurological function. This represents the first report of MRI findings in acute transverse myelitis after influenza vaccination.