The present study aims to assess the prevalence of autism spectrum disorder (ASD) in preschool and school-age children following a two-phase procedure. The screening phase was performed on a sample of 5555 children taking into account parent and teacher information. The individual assessment included the ADI-R, ADOS-2 and Wechsler scales. The estimated prevalence was 1.55% in preschoolers and 1.00% in school-age children. Between 1.84 and 2.59% of the children exhibited subclinical diagnosis. The male-to-female ratio was around 4:1. Most of the children exhibited mild and moderate nuclear symptoms, and the girls showed less severe communication problems. Previous diagnosis was found in 62–71% of the children. Prevalence estimates are close to the 1% international ratings and much higher than previous national reports suggested.