Internationally, research on psychiatric intensive care units (PICUs) commonly reports results from demographic studies such as criteria for admission, need for involuntary treatment, and the occurrence of violent behaviour. A few international studies describe the caring aspect of the PICUs based specifically on caregivers' experiences. The concept of PICU in Sweden is not clearly defined. The aim of this study is to describe the core characteristics of a PICU in Sweden and to describe the care activities provided for patients admitted to the PICUs. Critical incident technique was used as the research method. Eighteen caregivers at a PICU participated in the study by completing a semistructured questionnaire. In-depth interviews with three nurses and two assistant nurses also constitute the data. An analysis of the content identified four categories that characterize the core of PICU: the dramatic admission, protests and refusal of treatment, escalating behaviours, and temporarily coercive measure. Care activities for PICUs were also analysed and identified as controlling - establishing boundaries, protecting - warding off, supporting - giving intensive assistance, and structuring the environment. Finally, the discussion put focus on determining the intensive aspect of psychiatric care which has not been done in a Swedish perspective before. PICUs were interpreted as a level of care as it is composed by limited structures and closeness in care.