A previously healthy and developmentally normal 5-year-old boy from Saudi Arabia presented with a 3 mo history of abnormal behavior in the form of hyperactivity, decreased social engagement, episodic aggressiveness, decreased cognitive performance in school, inappropriate laughter and talking to himself, insomnia, tics and other stereotyped behaviors. His clinical neurological examination did not reveal any focal neurological deficits, but a prolonged electroencephalogram showed non-specific slowing over the right hemisphere. A diagnosis of encephalopathy was entertained, and he underwent testing for infectious, toxic, metabolic and mitochondrial disorders. Magnetic resonance imaging of the brain and cerebrospinal fluid studies were all normal. However, a comprehensive evaluation of paraneoplastic antibodies from cerebrospinal fluid demonstrated the presence of antibodies against the alpha-3 subunit of the neuronal nicotinic acetylcholine receptor, a finding also documented by tests from two different laboratories. An extensive diagnostic investigation that included a fludeoxyglucose-positron emission tomography scan for malignancies showed negative results. The patient was treated with plasmapheresis treatments (five exchanges) that resulted in a significant improvement in his clinical picture. Following his initial treatments, the patient continued to receive monthly infusions of intravenous immunoglobulin. An evaluation at 5.5 mo after initiation of first treatment revealed a considerable improvement of his behavior and language abilities. The patient returned to his school where he has performed satisfactorily. To our knowledge, this is the first report of a child who developed sub-acute cognitive and neurobehavioral regression in association with the presence of a serum antibody to a neuronal ganglionic nicotinic acetylcholine receptor and who improved after immunotherapy.