Objective To give an overview of the clinical, radiological and laboratory characteristics of the choreoathetotic “relapse” following herpes simplex virus (HSV) encephalitis. Methods The charts of all patients treated at Szent Laszlo Hospital for HSV encephalitis between 1998 and 2014 were examined for possible signs of deterioration following unequivocal improvement during successful antiviral therapy. Clinical, laboratory and radiological data of these children were analysed. Results A “relapse” manifesting with choreoathetosis was diagnosed in 7 patients. All of them were under 2 years of age, representing nearly half of HSV encephalitis patients in this age group. Symptoms – choreoathetosis, orofacial dyskinesia, loss of appetite, insomnia, attention deficit, aggression, agitation, fever and seizures – developed between days 22 and 38 of herpes encephalitis. HSV DNA could not be found in any of the patients' cerebrospinal fluid by the time of the “relapse” however, signs of intrathecal antibody synthesis were present in nearly all of them. MR images showed no new lesions, the primary lesions were all in the chronic stage. Whatever was the reason, anti NMDAR antibodies could be shown in 2 patients. In one of them plasma exchange resulted in rapid improvement of both movement disorder, attention problems and epileptic spasms. Neither methylprednisolone pulse therapy, nor repeated course of antiviral treatment had convincing influence on the outcome. Choreoathetosis subsided in all children but all remained epileptic and mentally impaired. Conclusion Choroathetosis following HSV encephalitis is a rare clinical entity most likely due to immune mechanism. Children under 2 years of age are most commonly effected. Though the clinical course is resulting in epilepsy and cognitive impairment, plasma exchange – tried in a patient with NMDAR antibodies – may have a beneficial effect on the outcome. [ABSTRACT FROM AUTHOR]