Autism from 2 to 9 years of age. 
Autism represents an unusual pattern of development beginning in the infant and toddler years.  To examine the stability of autism spectrum diagnoses made at ages 2 through 9 years and identify features that predicted later diagnosis.  Prospective study of diagnostic classifications from standardized instruments including a parent interview (Autism Diagnostic Interview-Revised [ADI-R]), an observational scale (Pre-Linguistic Autism Diagnostic Observation Schedule/Autism Diagnostic Observation Schedule [ADOS]), and independent clinical diagnoses made at ages 2 and 9 years compared with a clinical research team's criterion standard diagnoses.  Three inception cohorts: consecutive referrals for autism assessment to (1) state-funded community autism centers, (2) a private university autism clinic, and (3) case controls with developmental delay from community clinics.  At 2 years of age, 192 autism referrals and 22 developmentally delayed case controls; 172 children seen at 9 years of age.  Consensus best-estimate diagnoses at 9 years of age.  Percentage agreement between best-estimate diagnoses at 2 and 9 years of age was 67, with a weighted kappa of 0.72.  Diagnostic change was primarily accounted for by movement from pervasive developmental disorder not otherwise specified to autism.  Each measure at age 2 years was strongly prognostic for autism at age 9 years, with odds ratios of 6.6 for parent interview, 6.8 for observation, and 12.8 for clinical judgment.  Once verbal IQ (P = .001) was taken into account at age 2 years, the ADI-R repetitive domain (P = .02) and the ADOS social (P = .05) and repetitive domains (P = .005) significantly predicted autism at age 9 years.  Diagnostic stability at age 9 years was very high for autism at age 2 years and less strong for pervasive developmental disorder not otherwise specified.  Judgment of experienced clinicians, trained on standard instruments, consistently added to information available from parent interview and standardized observation.