Earlier intervention improves outcomes for children with autism spectrum disorders (ASDs), but existing identification tools are at the limits of standardization with 18-month-olds. We assessed potential behavioural markers of ASD at 18 months in a high-risk cohort of infant siblings of children with ASD. Prospective data were collected using the Autism Diagnostic Observation Schedule (ADOS) and Autism Observation Scale for Infants (AOSI) on 155 infant siblings and 73 low-risk controls at 18 months. Infants were classified into three groups (ASD sibs, non-ASD sibs, controls) based on blind best-estimate diagnosis at age 3. Fisher's exact tests, followed by discriminant function analyses, revealed that the majority of informative ADOS items came from the social and behavioural domains, and AOSI items measuring behavioural reactivity and motor control contributed additional information. Findings highlight the importance of considering not only social-communication deficits, but also basic dimensions of temperament including state regulation and motor control when assessing toddlers with suspected ASD. (Contains 2 figures and 7 tables.)