Recognition of early signs of autism is crucial to early intervention. Signs of autism in the second year of life have already been established through retrospective home video studies of children diagnosed with autism. On the aim of having infants reaching intervention earlier than two years old, research has been conducted with a promising methodology. It consisted of following prospectively development of high risk infants. Since autism diagnosis is more frequent in younger siblings of children with autism, they are considered to be at increased risk for the disorder. On the other hand, infants without family history of autism are considered to be at low risk for autism. In the present prospective multiple case study, development of two high risk infants were followed during the first year and compared to development of two low risk infants. Interactions of these infants with an adult were recorded in video monthly. These videos were analyzed according to affective, interactive and coping categories. Historical narratives of interactions from 3 to 12 months were conducted as well. Results of categories analysis did not showed differences among infants of both groups. Interestingly, historical narratives showed that, between 8 and 12 month, one of the high risk infants had difficulties in matching his affect intensity with affect intensity of his interactive partner and also on tracking adults face during peek-a-boo game. At 21 months this infant was preventively referred to early intervention. Implications of using affective categories and affective matching in assessing high risk infants as well as in screening instruments are discussed.