First termed Dementia Infantilis by Theodore Heller in 1908, Childhood Disintegrative Disorder (CDD) has had a history longer than that of Autistic Disorder. Presently, CDD is classified as a Pervasive Developmental Disorder in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders. The characteristics most often cited as distinguishing CDD from Autistic Disorder, another one of the Pervasive Developmental Disorders, is the age of onset and evidence of normal development prior to the presence of symptomatology. Otherwise, the behavioral symptoms of CDD and Autistic Disorder are strikingly similar. The purpose of this article is to provide a historical background on CDD, examine the evolution of diagnostic criteria, review the existing literature pertaining to the disorder, and, finally, to draw conclusions regarding the validity of CDD as a distinct diagnosis with reference to current and alternative classification approaches.