The use of staffing agencies and agency social workers in statutory social work has not been given much attention from scholars, despite the extensive implications it may have for the organizations, employees and clients. By analysing the result of structured telephone interviews with representatives from 102 Swedish municipalities, this article brings some knowledge to this scarcely researched field. The aim is to map the extent of use of agency social workers, to analyse the reasons behind the use, and to identify any potential variation between municipalities and branches of social work. The findings show that the use of agency social workers is common, widespread and surprisingly unrelated to socioeconomic, geographic and demographic characteristics of the municipalities. The most common reasons for hiring agency workers appear to be related to achieving numerical stability of staffing levels, despite vacancies and recruitment difficulties. Although there were few detected differences between municipalities, there were great differences between branches of social work. It was far more common for child welfare units to contract agency social workers compared to social assistance and substance abuse units. Considering the vulnerability of the children that come into contact with the social services and the importance of continuity in these cases, this is a cause for concern. In addition, practically all hired agency social workers were contracted to investigate and assess the clients’ needs, i.e. partaking in the exercise of public authority, which adds legal and professional complexity.