To be diagnosed with autistic disorder at least one sign (each) from parts A, B, and C must be present plus at least six overall. Those meeting fewer criteria are diagnosable as PDD NOS.

A. Qualitative impairments in reciprocal social interaction:

1. Marked impairment in the use of multiple non-verbal behaviors such as eye-to-eye gaze, facial expression, body posture, and gestures to regulate social interaction.
2. Failure to develop peer relationships appropriate to developmental level.
3. Lack of spontaneous seeking to share enjoyment, interests, or achievements with others.
4. Lack of socio-emotional reciprocity.

B. Qualitative impairments in communication:

1. A delay in, or total lack of the development of spoken language (not accompanied by an attempt to compensate through alternative modes of communication such as gesture or mime).
2. Marked impairment in the ability to initiate or sustain a conversation with others despite adequate speech.
3. Stereotyped and repetitive use of language or idiosyncratic language.
4. Lack of varied spontaneous make-believe play or social imitative play appropriate to developmental level.

C. Restricted, repetitive, and stereotyped patterns of behavior, interest, or activity:

1. Encompassing preoccupation with one or more stereotyped and restricted patterns of interest, abnormal either in intensity or focus.
2. An apparently compulsive adherence to specific nonfunctional routines or rituals.
3. Stereotyped and repetitive motor mannerisms (e.g. hand or finger flapping, or twisting, or complex whole body movements).
4. Persistent preoccupation with parts of objects.

Abnormal or impaired development prior to age three manifested by delay or abnormal functioning in at least one of the following areas: (1) social interaction, (2) language as used in social communication, or (3) symbolic or imaginative play.

Source: The Diagnostic and Statistical Manual, 4th Edition, American Psychiatric Association, (1994).