An acute disintegrative disorder in a child with acute hepatitis B virus (HBV) infection is described. Both hepatitis B surface antigen (HBsAg) and HBV-DNA were detected in cerebrospinal fluid (CSF) by means of enzyme-linked immunosorbent assay (ELISA) and the polymerase chain reaction (PCR) technique respectively. A markedly elevated level of CSF adenylate kinase (AK), which normalised as the patient recovered spontaneously, suggested an organic brain disorder. Demonstration of intra-blood-brain barrier production of IgG supported the possibility of local infection by HBV within the central nervous system.