Summary: This paper describes the results of an evaluation study of a prototype for the visualization of time-oriented medical data. Subjects were nine physicians. The prototype combines well-known visual representation techniques and extensive interaction techniques. The aim of the study was to assess the system’s usability and whether the prototype solved relevant problems of physicians in hospitals. It was found that one of the great advantages of the system was that it allowed physicians to see the development of the patients at one glance. It was also shown that users clearly preferred an easy to learn and understand design and familiar visualizations.