This study examines the following properties of the Modified Checklist for Autism in Toddlers (M-CHAT) in an unselected low-risk sample: (a) the maximum age for screen administration; (b) the positive screen rate in the absence of follow-up telephone interviews and; (c) the distributional properties of positive screens. Data came from a prospective cohort study (n = 1,604). Results suggest that the M-CHAT can appropriately be administered to children aged 20-48 months. Documented explanations provided by mothers during screening, appear to effectively identify potential screen misclassifications in the absence of the follow-up telephone interviews. This further emphasizes the importance of clinician expertise in verifying positive M-CHAT screens. Results have implications for the administration of the M-CHAT in clinical and research settings. (Contains 5 tables.)