Childhood Disintegrative Disorder (CDD), a clinical syndrome distinct from childhood autism, is a rare unremittingly pervasive developmental disorder resulting from disintegration of mental functions and progressive neurological abnormality. This rare condition is characterized by regression or loss of previously acquired language and social skills after a period of at least 2 years of normal development. This report presenting a case of a 10-year-old boy who presented with normal development until 3-4 years of age followed by gradually developmental deterioration in previously acquired social skills, language and intellectual functions with aberrant behaviors suggestive of childhood disintegrative disorder. This case is reported as a very rare case and there is no previous official report in Thailand.

Keywords: Childhood disintegrative disorder, Heller’s syndrome, Dementia infantilis,Pervasive developmental disorder