The paper presents the role of a social worker within a mobile unit, based on the Dutch model entitled Flexible Assertive Community Treatment (FACT). The model is intended for the patients with severe mental disorders who need community-based care following their hospital treatment in order to be able to complete their rehabilitation and learn to live with their disorder. The main objective of the paper is to present the FACT's mobile unit and show the previous experiences and practices related to community treatment implementation. The approach within the unit is multidisciplinary, with the participation of a social worker whose role is defined in the paper. The role is presented based on the experiences in the field of psychiatry and the assertive treatment, as well as on the utilisation of the case management theoretical perspective. Social work as a profession has got a multiple role in psychiatry and the social workers can also be case managers for their knowledge and skills. The mobile unit operates on the same principle. In psychiatry, social work is clearly defined and social workers have considerable experience of assertive treatment practice. Such knowledge should also be applied to flexible assertive treatments, in addition to further training and the social workers' involvement in terms of providing a more precise explanation of the roles in order for the provided services to be of high quality and taking into consideration the well-being of the patients.