The study examined the properties of the Screening Tool for Autism in Two-Year-Olds (STAT) for children under 24 months. The STAT provides a standard context for observing social-communicative behavior in play, imitation, and communication. Seventy-one children received the STAT between 12 and 23 months of age and a follow-up diagnostic evaluation after 24 months. All had an older sibling with an autism spectrum diagnosis (n = 59) or had been referred for evaluation for concerns about autism (n = 12). Signal detection analysis resulted in a cut score of 2.75 for this sample, which yielded a sensitivity of 0.95, specificity of 0.73, positive predictive value of 0.56, and negative predictive value of 0.97. False positives were highest for the 12- to 13-month-old age group; STAT screening properties were improved when the sample was limited to children 14 months and older. Implications for using the STAT with children under 24 months are discussed. (Contains 1 figure and 3 tables.)