BACKGROUND: Pediatricians, neurologists, and geneticists are important sources for autism surveillance, screening, and referrals, but practical time constraints limit the clinical utility of behavioral observations. We analyzed behaviors under favorable conditions (ie, video of autism evaluations reviewed by experts) to determine what is optimally observable within 10-minute samples, asked for referral impressions, and compared these to formal screening and developmental testing results. METHODS: Participants (n = 42, aged 15 to 33 months) were typically developing controls and children who screened positive during universal autism screening within a large community pediatric practice. Diagnostic evaluations were performed after screening to determine group status (autism, language delay, or typical). Licensed psychologists with toddler and autism expertise, unaware of diagnostic status, analyzed two 10-minute video samples of participants’ autism evaluations, measuring 5 behaviors: Responding, Initiating, Vocalizing, Play, and Response to Name. Raters were asked for autism referral impressions based solely on individual 10-minute observations. RESULTS: Children who had autism showed more typical behavior (89% of the time) than atypical behavior (11%) overall. Expert raters missed 39% of cases in the autism group as needing autism referrals based on brief but highly focused observations. Significant differences in cognitive and adaptive development existed among groups, with receptive language skills differentiating the 3 groups. CONCLUSIONS: Brief clinical observations may not provide enough information about atypical behaviors to reliably detect autism risk. High prevalence of typical behaviors in brief samples may distort clinical impressions of atypical behaviors. Formal screening tools and general developmental testing provide critical data for accurate referrals.