The present study aimed to identify the cumulative incidence of autism spectrum disorder (ASD) in a thoroughly screened population and to examine the behavioral and motor characteristics observed in children with ASD at the age of 18 months. Subjects were 1067 children who underwent a screening assessment for ASD at the routine 18-months health checkup. By the age of 6 years, 3.1% (4.3% of boys and 2.0% of girls) were diagnosed as having ASD by their attending pediatricians. Higher rate of difficulties in motor skills and social and communication skills had been reported in children with ASD at 18 months of age. This study showed that careful community-based screening system may be helpful in detecting ASD at early age.