Background: Increases in the incidence and prevalence of autistic spectrum disorders have been reported recently in the United States and Europe, but there are only a few reports on the trend of this problem in Thailand.
Objectives: To study trend in autistic spectrum disorder patients and to find the factors that correlate with the incidence of this disorder at Queen Sirikit National Institute of Child Health.
Material and Method: A hospital-based prospective trend study was conducted in patients aged less than 12 years old, who attended the Child and Adolescent Department, Queen Sirikit National Institute of Child Health, Bangkok, form January 1998 to December 2002. Autistic spectrum disorders were defined as autistic disorder, pervasive developmental disorder-not otherwise specified (PDD-NOS), and Asperger’s disorder.
Related factors were recorded and analyzed. Aged-adjusted incidence rate in both sexes and proportion rates of related factors were calculated by using the direct method. Standard t-test and correlation coefficient were performed to test for statistically significant difference in the prevalence rate in each group.
Results: There were 610 new patients aged less than 12 years who displayed behavior consistent with autistic spectrum disorders. The increasing trend in the incidence of autistic spectrum disorder is observed (r = 0.935, p = 0.02). The incidence rate of autistic spectrum disorders among out patients less than 12 years has increased from 1.43 per 10,000 in 1998 to 6.94 per 10,000 in 2002. The correlation factors “an only one child in the family” is observed (r = 0.9, p = 0.038).
Conclusion: During the 5 years of the study, a significant increase in the incidence of autistic spectrum disorders was observed at Queen Sirikit National Institute of Child Health. Increase in the incidence of ASD in this center cannot explain by any solid evidence from the present study.

Keywords: Trend, Autistic spectrum disorder