The factors underlying acute infantile hemiplegia are seldom identified. Coxsackie A9 focal encephalitis was documented for the first time in a 3-month-old infant with fever, hemiconvulsions, and hemiplegia followed by a static motor deficit and epilepsy. It has been suggested that the acute infantile hemiplegia associated with encephalitis results from an arteritis or venous sinus thrombosis with subsequent cerebral infarction. However, this was not observed in our patient. Rather, a series of brain scans, computerized tomograms, and a cerebral angiogram clearly documented the evolution of a focal necrotizing encephaloclastic process resulting in a porencephalic cyst. Serial cerebrospinal fluid viral cultures were necessary to isolate the etiologic agent (Coxsackie A9). The infant did not have a neutralizing antibody response to the infecting viral agent despite an apparently intact immune system, which possibly may be explained by the development of immune tolerance or an insufficient amount of infecting viral antigen. This emphasizes that serologic studies alone may not be adequate to document an acute central nervous system viral infection. This patient also typifies the poor prognosis in infants presenting with acute hemiplegia, fever, and convulsions in the absence of cerebrovascular occlusion.