This article presents a longitudinal study with four children with autism who were exposed to a humanoid robot over a period of several months. The longitudinal approach allowed the children time to explore the space of robot-human, as well as human-human interaction. Based on the video material documenting the interactions, a quantitative as well as qualitative analysis was conducted. The quantitative analysis showed an increase in duration of pre-defined behaviours towards the later trials. A qualitative analysis of the video data, observing the children's activities in their interactional context, revealed further aspects of social interaction skills (imitation, turn-taking and role-switch) and communicative competence that the children showed. The results clearly demonstrate the need for and benefits of long-term studies in order to reveal the full potential of robots in the therapy and education of children with autism.