Inclusion Criteria:

• Able to walk
• Diagnosis of Autistic Disorder, Asperger's Disorder, or PDD-NOS
• Have a score greater than or equal to (>) 8 on the sum of items 1A, 2, 3 and 5 of the Compulsions Subscale of the Revised CYBOCS.
• Have a rating of at least moderate behavioral disturbance based on the modified Clinical Global Impression-Severity of Illness score (CGI-S) at the time of screening (See description below).
• Be free of psychotropic medication for at least one month for fluoxetine, two weeks for other SSRIs and neuroleptics, and for 5 days for stimulants prior to baseline ratings.
• Be judged reliable for medication compliance and agree to keep appointments for study contacts and tests as outlined in the protocol (both subject and guardian(s)).

Exclusion Criteria:

• Medical contraindications to therapy with SSRIs
• Prior exposure to citalopram (or escitalopram) of sufficient dose or duration to determine response status
• History of treatment failure to a clinically adequate trial of two select SSRIs
• Diagnosis of Rett's Disorder or Childhood Disintegrative Disorder
• Uncontrolled epilepsy, with a seizure within past 6 months
• Child weighs less than (<) 15 kg at screening contact.
• Pregnancy
• Presence of chronic medical conditions that might interfere with study participation or where study participation would be contraindicated
• Clinically significant abnormal baseline laboratory testing
• History of bipolar disorder or manic episode induced by antidepressant exposure
• Documented need for ongoing psychotropic medications besides study medication (with the exception of stable dose (at least 3 month) anti-convulsants for seizures).
• Concomitant medication that would interfere with participation in the study.
• Recent (< 2 months) initiation of behavior therapy (such as parent training, applied behavior analysis or behavior modification) in a clinic, with a private practitioner or in a school program. Participants who are in an established behavior therapy program (defined as greater than (>) 2 months for clinic or private practitioner or greater than (>) 1 month for school program) can be included in the study. Families will be asked not to initiate any new behavior therapy during the study.