This study examined whether the interaction of severely depressed patients and a psychiatrist was related to the course of depression during hospitalization. Interactional Processes were defined on the basis of directly observed behaviour displayed during an interview and by the use of ethological methods. The behavioural structure of the interaction could be described by five factors. The severity of depression and the level of the behaviour factors were assessed just after admission (at baseline) and 10 weeks later. Patients were divided a posteriori into non-improved patients (N = 13) and improved patients (N = 18). Patients who did not improve displayed more 'speaking-effort' (looking, gesticulating, head movements during speech) and less 'active-listening' (head movements and intensive body touching during listening to the psychiatrist) than those who did improve. These factors increased over time in the improved patients but not in the non-improved patients suggesting that they may play a role in the maintenance of depression. The study illustrates the possible value of an ethological approach in the study of interactional processes