An 18-month-old boy presented to the emergency department with a rash on the left lower extremity. He was fully vaccinated, with routine vaccines at 12 months of age administered in the left thigh. The influenza vaccine was administered 1 week prior to presentation in the left buttock, after which his parents noted a red patch near the site of vaccination. The patch became raised and spread down the left thigh over the course of several days. The rash did not bleed, ulcerate, weep, or appear itchy or painful, though the patient did seem uncomfortable when getting dressed. He had no known sick contacts at day care, exposure to herpes simplex virus, or exposure to new topical products or clothing. His pediatrician prescribed cephalexin and mupirocin for a presumed cutaneous bacterial infection. The eruption continued to spread down his left thigh over the next 2 days, at which point the pediatrician advised the family to go to the emergency department.