Inclusion Criteria:

• 1) Male or female of any race or ethnicity. 2) Ambulatory status (outpatient or day-treatment) at time of randomization. 3) Age 5-17 years inclusive at the time of consent 4) Subjects will meet criteria for the diagnosis of Autistic Disorder, Asperger’s Disorder, or PDD-NOS as determined by ADI-R administered by raters who are trained to research reliability, and confirmed by an experienced and reliable clinician using DSM-IV-TR criteria.

  5) Subjects must also have a score greater than 7 on the first 3 items of the Compulsions Subscale of the Revised CYBOCS.

  6) Subjects must have a rating of at least moderate (ratings of 4 or greater) behavioral disturbance based on the modified Clinical Global Impression-Severity of Illness score (CGI-S) at the time of screening (See description below).

  7) Subject must demonstrate a mental age >18 months as determined by the Vineland Adaptive Behavior Scales.

  8) Subjects must 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.

  9) Subjects and their parents (guardians) must be judged reliable for medication compliance and must agree to keep appointments for study visits and tests as outlined in the protocol.

  10) Subjects enrolled in an applied behavior analytic educational program for children with autism will be included in the Experimental FBA study only. **

Exclusion Criteria:

• 1) Subjects who have medical contraindications to therapy with SSRIs as determined by medical history or known allergy.

  2) Concomitant medication that would interfere with participation in the study. 3) Subjects with a prior history of treatment failure to a clinically adequate trial of two selective serotonin reuptake inhibitors (SSRI).

  4) Subjects with a previous diagnosis of Rett’s Disorder or Childhood Disintegrative Disorder.

  5) Subjects with documented need for other ongoing psychotropic medications besides study medication (with the exception of stable dose (at least 3 month) anti-convulsants for seizures).

  6) Subjects with uncontrolled epilepsy (seizure within past 6 months). 7) Presence of chronic medical conditions that might interfere with study participation or where study participation would be contraindicated, or clinically significant abnormal baseline laboratory testing.

  8) Subject with known personal history of bipolar disorder or prior manic episode induced by antidepressant exposure.

  9) Subjects with a prior history of an exposure to citalopram of sufficient dose or duration to determine response status.

  10) Subjects that are not enrollment in an applied behavior analytic educational program for children with autism will be excluded from the Experimental FBA study only.

  *Cases which do not meet exact inclusion or exclusion criteria but questioned as appropriate by the site PI will be reviewed by the multi-site STAART protocol committee for final decision about study eligibility. The protocol committee will take care to insure that the exception 1) is consistent with the subject’s welfare and 2) does not compromise the scientific purpose of the study. Such exceptions will be coded as an approved protocol deviation and will be communicated to the local Institutional Review Board in a timely manner.