To determine whether the Modified Checklist for Autism in Toddlers (M-CHAT) in conjunction with the routine 18-month health check-up identifies Japanese toddlers with autism spectrum disorder (ASD). Two-stage screening using the M-CHAT was conducted with 1,851 children attending the check-up. Final ASD diagnosis was confirmed at age =3 years. Screening identified 20/51 children with ASD: 12/20 true positives were developmentally delayed, whereas 16/22 false negatives were high-functioning. Sensitivity was 0.476, specificity 0.986, positive predictive value 0.455, and likelihood ratio 33.4 for children with ASD. With a few modifications, M-CHAT screening successfully detected toddlers with ASD with and without developmental delay and is a promising screening tool to complement existing community surveillance.