The behaviours of infants were observed using home videos, in an attempt to identify social difficulties characteristic of infants with autistic disorder. Three groups of infants were analysed: 15 infants who had later been diagnosed with autism, 15 infants who had a developmental or language delay, and 15 typically developing infants. Social behaviours were coded using both quantitative and qualitative measures. The principal discriminating items between the groups were found to be ‘peer interest’, ‘gaze aversion’, ‘anticipatory postures’, and ‘proto-declarative showing’. The results suggest that these children later diagnosed with autism are clinically distinct from their peers before the age of two years, and that there are clearly observable behaviours which are important predictors of autistic disorder in pre-verbal children.