The Social Attention and Communication Study involved the successful implementation of developmental surveillance of the early markers of autism spectrum disorders in a community-based setting. The objective in the current study was to determine the most discriminating and predictive markers of autism spectrum disorders used in the Social Attention and Communication Study at 12, 18 and 24 months of age, so that these could be used to identify children with autism spectrum disorders with greater accuracy. The percentage of "yes/no" responses for each behavioural marker was compared between children with autistic disorder (n = 39), autism spectrum disorder (n = 50) and developmental and/or language delay (n = 20) from 12 to 24 months, with a logistic regression also conducted at 24 months. Across all ages, the recurring key markers of both autistic disorder and autism spectrum disorder were deficits in eye "contact" and "pointing", and from 18 months, deficits in "showing" became an important marker. In combination, these behaviours, along with "pretend" play, were found to be the best group of predictors for a best estimate diagnostic classification of autistic disorder/autism spectrum disorder at 24 months. It is argued that the identified markers should be monitored repeatedly during the second year of life by community health-care professionals. (Contains 3 tables, 7 figures and 7 notes.)