This study aimed to determine the optimal cut-off for autism spectrum disorder (ASD) screening in 66-month-old children, and to explore the distribution of ASD screening and diagnosis in Taiwan. The Taiwan Birth Cohort Study dataset was used (N = 20,095). The Modified Checklist for Autism in Toddlers (M-CHAT) cut-off point of 13/14 was considered optimal for screening of children at 66 months. More children were diagnosed with ASD in urban areas. Parents of children diagnosed with ASD had a higher level of education, but parents of children with a lower level of education were screened as being at higher risk of ASD. Urban disparity and parental level of education effected parental awareness of the illness and the rate of ASD diagnosis.