OBJECTIVE Communication disorders have been reported following severe traumatic brain injury. However, we have little information about patient behaviour during dyadic interaction. Here, we analyzed conversation at the rehabilitation and chronic phase post traumatic brain injury (TBI), to define the main mechanisms of verbal and non-verbal communication disorders and relationship with other cognitive difficulties. METHODS Sixteen patients were evaluated at the rehabilitation phase (2-12 months) and 18 at the chronic phase (after 2 years) following severe TBI. They were compared with equivalent groups of matching (gender, age, education level) control subjects. We used the Lille Communication Test, which comprises three parts: participation to communication (greeting, attention, engagement), verbal communication (verbal comprehension, speech outflow, intelligibility, word production, syntax, verbal pragmatics, verbal feedback) and non-verbal communication (understanding gestures, affective expressivity, producing gestures, pragmatics, non-verbal feedback). We also investigated executive functions (Stroop test, trail-making test, categorical evocation), language (Montreal-Toulouse protocol) and behaviour (Neurobehavioural Rating Scale). Verbal communication disorders were relatively equivalent at the rehabilitation and chronic phases. RESULTS Patients were impaired (P < or = 0.01) in their participation to communication, especially in greeting behaviour. Verbal communication was mostly affected by difficulties in producing fluent and intelligible language and using pragmatics (responding to open questions, presenting new information and introducing new themes, organizing discourse and adapting to interlocutor knowledge). Non-verbal communication was impaired by difficulties in using pragmatics (mostly adapted prosody). Participation and verbal communication correlated with the executive functions, language and behavioural assessment. CONCLUSIONS Disorders of social communication justify systematic assessment in patients with TBI.