Children appear to develop normally for the first two years of life, but then lose skills in areas such as language, play, and bowel control and manifest impaired social interaction and communication associated with "restrictive, repetitive, stereotyped" behaviors. Apparently normal development for at least the first 2 years after birth as manifested by the presence of age-appropriate verbal and nonverbal communication, social relationships, play, and adaptive behavior.

Clinically significant loss of previously acquired skills (before age 10 years) in at least two of the following areas: expressive or receptive language, social skills or adaptive behavior, bowel or bladder control - play - motor skills .

Abnormalities of functioning in at least two of the following areas: qualitative impairment in social interaction (e.g., impairment in nonverbal behaviors, failure to develop peer relationships, lack of social or emotional reciprocity), qualitative impairments in communication (e.g., delay or lack of spoken language, inability to initiate or sustain a conversation, stereotyped and repetitive use of language, lack of varied make-believe play) - restricted, repetitive, and stereotyped patterns of behavior, interests, and activities, including motor stereotypies and mannerisms.