This study assessed the psychometric properties of the Modified Checklist for Autism in Toddlers (M-CHAT) and the Social Communication Questionnaire (SCQ) in a sample of preschool children referred for possible pervasive developmental disorders (PDDs). The sample consisted of 82 children between the ages of 18 and 70 months (54 with a PDD diagnosis and 28 with non-PDD diagnoses). M-CHAT scores were analyzed for 56 children aged 18-48 months old and SCQ scores were analyzed for 65 children aged 30-70 months old. Optimal sensitivity and specificity were achieved using the cutoff score of any three items on the M-CHAT and lowering the cutoff score of the SCQ. The diagnostic agreement of both instruments was also compared in an overlapping subsample of 39 children aged 30-48 months. Overall, the M-CHAT and SCQ appear to more accurately classify children with PDDs who have lower intellectual and adaptive functioning. (Contains 5 tables.)