In a case of Mycoplasma pneumoniae encephalopathy in a 7-year-old girl, the microorganism was identified by serologic and pharyngeal-swab polymerase chain reaction methods. Failure to detect or isolate the pathogen in the cerebrospinal fluid and the significant rapid clinical recovery of the patient after intravenous immunoglobulin administration suggest an immune-mediated mechanism. In critically ill patients with M. pneumoniae encephalopathy, prompt intravenous immunoglobulin treatment may be beneficial and can be considered as a primary or alternative therapeutic approach, especially when use of corticosteroids is insecure.