A 7-year-old female presented with Mycoplasma pneumoniae pneumonitis and a progressive ascending limb paralysis. She developed severe respiratory distress, requiring ventilation, and became apparently unresponsive with fixed dilated pupils. Peripheral nerves were inexcitable in nerve-conduction studies. Magnetic resonance imaging of the brain revealed evidence of extensive demyelination. Anti-GM1 immunoglobulin M antibody titers were raised. She improved after a second course of intravenous immunoglobulin and eventually made a full recovery.