A 19-year-old Chinese man presented with 3 days of acute onset of fever, headache, convulsion, encephalopathy, nausea, and vomiting. Meningitis was considered, but cerebrospinal fluid testing results were normal. After 48 hours, the patient’s mental status and ability to participate in an examination improved, and a Montreal Cognitive Assessment yielded a score of 19 of 30 points, with deficits in orientation, attention, language, memory, and visuospatial function. Diffuse hyporeflexia, distal muscle atrophy, and weakness were observed. Also noted were dysarthria, bradykinesia, postural tremor, truncal and limb ataxia, and marked gait instability. On a review of the patient’s medical history, clinicians found that extrapyramidal symptoms had gradually developed in the preceding 6 years, and weakness had been progressive for 2 years. The patient had also experienced 3 similar encephalitic episodes in the 21 months prior to admission. These episodes lasted between 2 and 7 days each and were treated with acyclovir and glucocorticoids each time. A broad diagnostic workup revealed unremarkable results with respect to thyroid function; lactate, copper and ceruloplasmin levels; red blood cell morphology; leukocyte lysosomal enzyme activities; autoimmune encephalitis antibodies; and rubella, cytomegalovirus, and herpes simplex virus serologic test results. Nerve conduction testing revealed length-dependent sensorimotor mixed axonal and demyelinating polyneuropathy and neurogenic changes on electromyography. A cranial magnetic resonance image showed moderate cerebral and cerebellar atrophy and notable leukoencephalopathy (Figure 1). Comprehensive gene detection, including whole-exon sequencing, and dynamic mutation detection for spinocerebellar ataxia, Friedreich ataxia, and fragile X–associated tremor/ataxia syndrome genes were performed and did not reveal any pathogenic mutation. Double immunofluorescence staining showed ubiquitin-positive intranuclear inclusions in the dermal cells (Figure 2A). Electron microscopy also revealed intranuclear inclusions in the nuclei of the dermal cells (Figure 2B). Thus, the patient was diagnosed with juvenile-onset neuronal intranuclear inclusion disease (NIID).