This article describes the interactions between patients and between patients and staff in 4 psychiatric wards. Two of the wards are acute admission units and 2 are for long-stay chronic patients. The hospital is diagnostically streamed so that not only does the average length of stay vary between wards but so, for one ward, does the predominant diagnostic group. Patient interactions were analysed in terms of 5 major interactional categories: individual verbal, individual nonverbal, group verbal, group nonverbal and physical. The results show that, although there was little change in the overall level of verbal interaction as a function of chronicity, there were large shifts in patient-patient and staff-patient interaction rates. Variations between the 4 wards on the 4 other interactional categories are explained in terms of the known diagnostic characteristics of the patients.