Early detection and intervention is essential for children with autism spectrum disorders (ASD). Therefore, we examined the reliability and validity of the Japanese version of the Modified Checklist for autism in toddlers (M-CHAT), a 23-item, yes-no questionnaire regarding early autistic symptoms completed by parents of children at 18-24 months of age. Herein, the reliability of the M-CHAT was investigated for children 4-20 months of age. The M-CHAT score (the number of failed items) was found to be significantly correlated among 24 mother-father pairs (Pearson's r = .933), representing good inter-rater reliability. The test-retest reliability was satisfactory, with 22 mothers providing almost equal M-CHAT scores on two different occasions (r = .990). Significant correlations were observed between the M-CHAT score and the Childhood Autism Rating Scale-Tokyo version score in 25 two-year-old children (r = .581), indicating good concurrent validity. The M-CHAT score was significantly higher in 20 children later diagnosed with ASD compared with reference children (n = 1167), revealing sufficient discriminant validity. A short version of the M-CHAT using 9 items was proposed and effectively differentiated children with ASD from reference children. The efficacy of the Japanese version of the M-CHAT was demonstrated for first-level screening in the general population. (C) 2011 Elsevier Ltd. All rights reserved.