The Pre-Linguistic Autism Diagnostic Observation Schedule (PL-ADOS) is a semistructured observation scale designed for use as a diagnostic tool for children less than 6 years old who are not yet using phrase speech and are suspected of having autism. The PL-ADOS takes approximately 30 minutes to administer and is appropriate for use with this population because of its emphasis on playful interactions and the use of toys designed for young children. Reliability studies indicated that both individual activity ratings and summary ratings could be reliably scored from videotaped assessments by naive raters. Additionally, PL-ADOS scores of nonverbal preschool-aged children referred for clinical diagnosis and classified on the basis of a diagnostic team's clinical judgment, clearly discriminated between autistic and nonautistic developmentally disabled children. The resulting diagnostic algorithm is theoretically linked to diagnostic constructs associated with ICD-10 and DSM-IV criteria for autism.