Recent years have seen a surge of interest in assessment instruments for diagnosing autism in children. Instruments have generally been developed and evaluated from a research perspective. The Autism Diagnostic Observation Schedule-Generic (ADOS-G), Autism Diagnostic Interview-Revised (ADI-R), and Gilliam Autism Rating Scale (GARS) have received considerable attention and are widely used. The objective of this study was to explore the diagnostic utility and discriminative ability of these tools using a clinical population of children referred to a specialty diagnostic clinic over a 3 year time span. The results indicated that the ADOS-G and ADI-R led to approximately 75 percent agreement with team diagnoses, with most inconsistencies being false positive diagnoses based on the measures. The GARS was generally ineffective at discriminating between children with various team diagnoses and consistently underestimated the likelihood of autism. The findings have important implications for the use of these measures in both research and clinical practice. (Contains 1 figure and 4 tables.)