The group dynamics visualizer is a solution to the problem healthcare workers are facing when working with social therapeutic groups. It is a well known problem that their reflection ability is affected negatively by the group dynamics. Healthcare workers become part of this groups dynamics and are no longer able to objectively observe and intervene. In this paper we describe the process of measuring group dynamics, a part of the 'group prevention plan'; more specific, giving feedback on the tension level of groups inside a psychiatric ward. A software application, 'group dynamics visualizer', is developed to measure and visualize the data. The visualization methods allow the user to quickly explore the data in order to find relations and abnormalities. This information is then used to prevent high-tension build up. Furthermore the software serves as an objective observation tool.