Objective: To assess the diagnostic stability of autism spectrum disorder in children less than three years. Material & Methods: Twenty children (16 boys, 4 girls) with a diagnosis of autistic disorder and Pervasive Developmental Disorder-Not Otherwise Specified (PDD-NOS) at age of 3 years or less as per DSM IV criteria and who had attained an age of 4 to 5 years were recruited from Pediatric Outpatient services. A Multi-disciplinary evaluation was done at diagnosis and follow up assessment was done around 4-5 years of age. Childhood Autism Rating Scale (CARS) used to assess symptom severity. Results: At Time 1, at the mean age of 2.4 (SD=4.94) years, 15 (75%) children were diagnosed as autistic and 5 (25%) children with PDD-NOS. At time 2, at the mean age of 3.9 (SD=9.49) years, 16 (80%) children were diagnosed with autism disorder and 2 (10%) children were diagnosed with PDD-NOS, and 2 (10%) children were found to be off the ASD spectrum. Looking at specific ASD diagnosis, the diagnostic stability of an Autistic Disorder was 12/15 (80%) and PDD-NOS was 0/5 (0%). Only 2 children went off the spectrum and 4 children developed the full symptoms at Time 2. Conclusion: The present study supports that a reliable diagnosis of autism can be made in children which would stand over time provided that expert clinicians are available. However, continued work is needed for the development of reliable and valid diagnostic tools.