All children aged 18-24 months in a defined geographical area were initially screened for autism, using 'Red Flag' criteria. All the children with one or more positive 'Red Flag' signs were further screened using Modified Checklist for Autism in Toddlers (M-CHAT) translated to Sinhala, followed by a comprehensive clinical assessment. Of a sample of 374 children, 'Red Flag' signs were positive in 28 (7.4%). Four children received a diagnosis of autism on clinical assessment giving a prevalence of 1.07% or 1 per 93 in the 18-24-month age group. Sensitivity of M-CHAT was only 25%, and specificity 70%. The high prevalence detected strongly justifies early community-based screening, but a culturally sensitive screening tool needs to be developed for Sri Lanka.