A severe pervasive developmental disorder which usually takes place before three years of age and a biological basis related to neurologic or neurophysiologic factors: An autistic disorder is characterized by qualitative impairment in reciprocal social interactions; such as, a lack of awareness of the existence of feelings of others, failure to seek comfort at times of distress, and of verbal and nonverbal communication.

Other characteristics of autistic disorders sometimes include cognitive impairment, "hyperactivitiy" or "hyporeactivity" to certain stimuli, stereotypic behaviors, neurological abnormalities; such as, seizures or altered muscle tone, sleeping or eating pattern abnormalities, and severe behavioral problems.